fix(session-start): support ECC_SESSION_RETENTION_DAYS opt-out + document env var (#2151) (#2163)

* fix(session-start): support ECC_SESSION_RETENTION_DAYS opt-out + document env var

The retention pass for *-session.tmp files (issue #2151) landed previously,
but the env var that controls it was undocumented in the README and rejected
falsy values (0, off, disabled), silently falling back to the 30-day default.
Users who want to keep all sessions for forensic or research workflows had no
way to opt out.

This patch:

- Extends getSessionRetentionDays() so 0|off|false|disabled|never|none disables
  pruning entirely (returns null sentinel; default behavior unchanged).
- Updates the call site in main() to skip pruneExpiredSessions when retention
  is null and emits a clear "[SessionStart] Pruning disabled via
  ECC_SESSION_RETENTION_DAYS" log line so the operator can tell pruning is off.
- Documents ECC_SESSION_RETENTION_DAYS in the README "Hook Runtime Controls"
  section alongside the other ECC_SESSION_* knobs.
- Adds three regression tests in tests/hooks/hooks.test.js covering opt-out
  via 0, opt-out via off, and garbage-value fallback to default 30.

Verification:
- node tests/hooks/hooks.test.js  — 240/240 green (incl. 3 new retention tests)
- node tests/run-all.js           — 2622/2622 green
- npx eslint scripts/hooks/session-start.js tests/hooks/hooks.test.js — clean
- node scripts/ci/validate-no-personal-paths.js — clean
- node scripts/ci/check-unicode-safety.js       — clean
- node scripts/ci/validate-hooks.js — 28 matchers validated
- node scripts/ci/validate-rules.js — 115 files validated

Fixes #2151

* docs(readme): list all ECC_SESSION_RETENTION_DAYS opt-out values + add Windows example

Address reviewer feedback on PR #2163:
- CodeRabbit and cubic both flagged that the README docs only listed 3 of 6
  opt-out values accepted by getSessionRetentionDays() (0, off, disabled),
  while the implementation also accepts false, never, none.
- cubic also flagged the missing Windows PowerShell example for the new
  variable, breaking the parallel structure of the existing
  ECC_CONTEXT_MONITOR_COST_WARNINGS example block.

Updated the README to:
- Spell out all six opt-out values (0, off, false, disabled, never, none)
  and clarify they "keep all sessions (disable pruning)".
- Add an ECC_SESSION_RETENTION_DAYS line to the Windows PowerShell example.

No behavior change. README only.

Verification:
- npx markdownlint README.md — clean
- npx eslint scripts/hooks/session-start.js tests/hooks/hooks.test.js — clean

README.md

@@ -479,6 +479,10 @@ export ECC_SESSION_START_MAX_CHARS=4000
 # Disable SessionStart additional context entirely for low-context/local-model setups
 export ECC_SESSION_START_CONTEXT=off
 
+# Session-tmp retention window in days (default: 30).
+# Set to 0, off, false, disabled, never, or none to keep all sessions (disable pruning).
+export ECC_SESSION_RETENTION_DAYS=14
+
 # Keep context/scope/loop warnings but suppress API-rate cost estimates
 export ECC_CONTEXT_MONITOR_COST_WARNINGS=off
 ```
@@ -487,6 +491,7 @@ Windows PowerShell:
 
 ```powershell
 [Environment]::SetEnvironmentVariable('ECC_CONTEXT_MONITOR_COST_WARNINGS', 'off', 'User')
+[Environment]::SetEnvironmentVariable('ECC_SESSION_RETENTION_DAYS', '14', 'User')
 ```
 
 ---

scripts/hooks/session-start.js

@@ -83,9 +83,22 @@ function dedupeRecentSessions(searchDirs) {
     .sort((left, right) => right.mtime - left.mtime || left.dirIndex - right.dirIndex);
 }
 
+/**
+ * Resolve session retention days from the ECC_SESSION_RETENTION_DAYS env var.
+ *
+ * @returns {number|null} The retention window in days, or `null` when the
+ *   user has explicitly opted out of pruning. Falsy/garbage values fall back
+ *   to {@link DEFAULT_SESSION_RETENTION_DAYS}.
+ */
 function getSessionRetentionDays() {
   const raw = process.env.ECC_SESSION_RETENTION_DAYS;
   if (!raw) return DEFAULT_SESSION_RETENTION_DAYS;
+
+  const normalized = String(raw).trim().toLowerCase();
+  if (['0', 'off', 'false', 'disabled', 'never', 'none'].includes(normalized)) {
+    return null;
+  }
+
   const parsed = Number.parseInt(raw, 10);
   return Number.isInteger(parsed) && parsed > 0 ? parsed : DEFAULT_SESSION_RETENTION_DAYS;
 }
@@ -526,9 +539,13 @@ async function main() {
   ensureDir(learnedDir);
 
   const retentionDays = getSessionRetentionDays();
-  const prunedSessions = pruneExpiredSessions(sessionSearchDirs, retentionDays);
-  if (prunedSessions > 0) {
-    log(`[SessionStart] Pruned ${prunedSessions} expired session(s) older than ${retentionDays} day(s)`);
+  if (retentionDays === null) {
+    log('[SessionStart] Pruning disabled via ECC_SESSION_RETENTION_DAYS');
+  } else {
+    const prunedSessions = pruneExpiredSessions(sessionSearchDirs, retentionDays);
+    if (prunedSessions > 0) {
+      log(`[SessionStart] Pruned ${prunedSessions} expired session(s) older than ${retentionDays} day(s)`);
+    }
   }
 
   const observerSessionId = resolveSessionId();

tests/hooks/hooks.test.js

@@ -5006,6 +5006,103 @@ async function runTests() {
     passed++;
   else failed++;
 
+  if (
+    await asyncTest('disables pruning when ECC_SESSION_RETENTION_DAYS=0', async () => {
+      const isoHome = path.join(os.tmpdir(), `ecc-start-prune-off-${Date.now()}`);
+      const sessionsDir = getCanonicalSessionsDir(isoHome);
+      fs.mkdirSync(sessionsDir, { recursive: true });
+      fs.mkdirSync(path.join(isoHome, '.claude', 'skills', 'learned'), { recursive: true });
+
+      const expiredFile = path.join(sessionsDir, '2026-01-01-keepme-session.tmp');
+      fs.writeFileSync(expiredFile, '# Old Session\n\nSHOULD STILL EXIST');
+      const ninetyDaysAgo = new Date(Date.now() - 90 * 24 * 60 * 60 * 1000);
+      fs.utimesSync(expiredFile, ninetyDaysAgo, ninetyDaysAgo);
+
+      try {
+        const result = await runScript(path.join(scriptsDir, 'session-start.js'), '', {
+          HOME: isoHome,
+          USERPROFILE: isoHome,
+          ECC_SESSION_RETENTION_DAYS: '0',
+        });
+
+        assert.strictEqual(result.code, 0);
+        assert.ok(fs.existsSync(expiredFile), 'Should keep all sessions when retention is opt-out=0');
+        assert.ok(result.stderr.includes('Pruning disabled via ECC_SESSION_RETENTION_DAYS'),
+          `Should log pruning disabled, stderr: ${result.stderr}`);
+        assert.ok(!result.stderr.includes('Pruned'), `Should not log any pruning, stderr: ${result.stderr}`);
+      } finally {
+        fs.rmSync(isoHome, { recursive: true, force: true });
+      }
+    })
+  )
+    passed++;
+  else failed++;
+
+  if (
+    await asyncTest('disables pruning when ECC_SESSION_RETENTION_DAYS=off', async () => {
+      const isoHome = path.join(os.tmpdir(), `ecc-start-prune-offstr-${Date.now()}`);
+      const sessionsDir = getCanonicalSessionsDir(isoHome);
+      fs.mkdirSync(sessionsDir, { recursive: true });
+      fs.mkdirSync(path.join(isoHome, '.claude', 'skills', 'learned'), { recursive: true });
+
+      const expiredFile = path.join(sessionsDir, '2025-12-15-keepme-session.tmp');
+      fs.writeFileSync(expiredFile, '# Forensic Session\n\nKEEP ME');
+      const sixtyDaysAgo = new Date(Date.now() - 60 * 24 * 60 * 60 * 1000);
+      fs.utimesSync(expiredFile, sixtyDaysAgo, sixtyDaysAgo);
+
+      try {
+        const result = await runScript(path.join(scriptsDir, 'session-start.js'), '', {
+          HOME: isoHome,
+          USERPROFILE: isoHome,
+          ECC_SESSION_RETENTION_DAYS: 'off',
+        });
+
+        assert.strictEqual(result.code, 0);
+        assert.ok(fs.existsSync(expiredFile), 'Should keep all sessions when retention is opt-out=off');
+        assert.ok(result.stderr.includes('Pruning disabled via ECC_SESSION_RETENTION_DAYS'),
+          `Should log pruning disabled, stderr: ${result.stderr}`);
+      } finally {
+        fs.rmSync(isoHome, { recursive: true, force: true });
+      }
+    })
+  )
+    passed++;
+  else failed++;
+
+  if (
+    await asyncTest('falls back to default retention when ECC_SESSION_RETENTION_DAYS is garbage', async () => {
+      const isoHome = path.join(os.tmpdir(), `ecc-start-prune-garbage-${Date.now()}`);
+      const sessionsDir = getCanonicalSessionsDir(isoHome);
+      fs.mkdirSync(sessionsDir, { recursive: true });
+      fs.mkdirSync(path.join(isoHome, '.claude', 'skills', 'learned'), { recursive: true });
+
+      const expiredFile = path.join(sessionsDir, '2026-01-01-pruneme-session.tmp');
+      fs.writeFileSync(expiredFile, '# Old Session\n\nDELETE ME');
+      const fortyDaysAgo = new Date(Date.now() - 40 * 24 * 60 * 60 * 1000);
+      fs.utimesSync(expiredFile, fortyDaysAgo, fortyDaysAgo);
+
+      try {
+        const result = await runScript(path.join(scriptsDir, 'session-start.js'), '', {
+          HOME: isoHome,
+          USERPROFILE: isoHome,
+          ECC_SESSION_RETENTION_DAYS: 'bogus-value',
+        });
+
+        assert.strictEqual(result.code, 0);
+        assert.ok(!fs.existsSync(expiredFile),
+          'Should fall back to default 30-day retention and prune the 40-day-old file');
+        assert.ok(result.stderr.includes('Pruned 1 expired session'),
+          `Should log pruning at default retention, stderr: ${result.stderr}`);
+        assert.ok(!result.stderr.includes('Pruning disabled'),
+          'Should NOT treat garbage as opt-out');
+      } finally {
+        fs.rmSync(isoHome, { recursive: true, force: true });
+      }
+    })
+  )
+    passed++;
+  else failed++;
+
   console.log('\nRound 55: session-start.js (newest session selection):');
 
   if (