docs+test: cycle #29 — document + lock text-mode vs JSON-mode exit divergence

Cycle #29 dogfood found a real pinpoint: cross-mode exit code divergence.

## The Pinpoint

Dogfooding the CLI revealed that unknown subcommand errors return different
exit codes depending on output mode:

  $ python3 -m src.main nonexistent-cmd                        # exit 2
  $ python3 -m src.main nonexistent-cmd --output-format json   # exit 1

ERROR_HANDLING.md documented the exit-code contract (1=parse, 2=timeout)
but did NOT explicitly state the contract applies only to JSON mode. Text
mode follows argparse defaults (exit 2 for any parse error), which
violates the documented contract when interpreted generally.

A claw using text mode with 'claw nonexistent' would see exit 2 and
misclassify as timeout per the docs. Real protocol contract gap, not
implementation bug.

## Classification

This is a DOCUMENTATION gap, not a behavior bug:
- Text mode follows argparse convention (reasonable for humans)
- JSON mode normalizes to documented contract (reasonable for claws)
- The divergence is intentional; only the docs were silent about it

Fix = document the divergence explicitly + lock it with tests.

NOT fix = change text mode exit code to 1 (would break argparse
conventions and confuse human users).

## Documentation Changes

ERROR_HANDLING.md:
1. Added IMPORTANT callout in Quick Reference section:
   'The exit code contract applies ONLY when --output-format json is
    explicitly set. Text mode follows argparse conventions.'
2. New 'Text mode vs JSON mode exit codes' table showing exact divergence:
   - Unknown subcommand: text=2, json=1
   - Missing required arg: text=2, json=1
   - Session not found: text=1, json=1 (app-level, identical)
   - Success: text=0, json=0 (identical)
   - Timeout: text=2, json=2 (identical, #161)
3. Practical rule: 'always pass --output-format json'

## Tests Added (5)

TestTextVsJsonModeDivergence in test_cross_channel_consistency.py:

1. test_unknown_command_text_mode_exits_2 — text mode argparse default
2. test_unknown_command_json_mode_exits_1 — JSON mode contract normalized
3. test_missing_required_arg_text_mode_exits_2 — same for missing args
4. test_missing_required_arg_json_mode_exits_1 — same normalization
5. test_success_path_identical_in_both_modes — success exit identical

These tests LOCK the expected divergence so:
- Documentation stays aligned with implementation
- Future changes (either direction) are caught as intentional
- Claws trust the docs

## Test Status

- 217 → 222 tests passing (+5)
- Zero regressions

## Discipline

This cycle follows the cycle #28 template exactly:
- Dogfood probe revealed real friction (test said exit=2, docs said exit=1)
- Minimal fix shape (documentation clarification, not code change)
- Regression guard via tests
- Evidence-backed, not speculative

Relationship to #181:
- #181 fixed env.exit_code != process exit (WITHIN JSON mode)
- #29 clarifies exit code contract scope (ONLY JSON mode)
- Both establish: exit codes are deterministic, but only when --output-format json

---

Classification (per cycle #24 calibration):
- Red-state bug? ✗ (behavior was reasonable, docs were incomplete)
- Real friction? ✓ (docs/code divergence revealed by dogfood)
- Evidence-backed? ✓ (test suite probed both modes, found the gap)

Source: Jobdori cycle #29 proactive dogfood — in response to Clawhip nudge
for pinpoint hunting. Found that text-mode errors return exit 2 but
ERROR_HANDLING.md implied exit 1 was the parse-error contract universally.

ERROR_HANDLING.md

@@ -10,12 +10,26 @@ After cycles #178–#179 (parser-front-door hole closure), claw-code's error int
 
 Every clawable command returns JSON on stdout when `--output-format json` is requested.
 
+**IMPORTANT:** The exit code contract below applies **only when `--output-format json` is explicitly set**. Text mode follows argparse conventions and may return different exit codes (e.g., `2` for argparse parse errors). Claws consuming claw-code as a subprocess MUST always pass `--output-format json` to get the documented contract.
+
 | Exit Code | Meaning | Response Format | Example |
 |---|---|---|---|
 | **0** | Success | `{success fields}` | `{"session_id": "...", "loaded": true}` |
 | **1** | Error / Not Found | `{error: {kind, message, ...}}` | `{"error": {"kind": "session_not_found", ...}}` |
 | **2** | Timeout | `{final_stop_reason: "timeout", final_cancel_observed: ...}` | `{"final_stop_reason": "timeout", ...}` |
 
+### Text mode vs JSON mode exit codes
+
+| Scenario | Text mode exit | JSON mode exit | Why |
+|---|---|---|---|
+| Unknown subcommand | 2 (argparse default) | 1 (parse error envelope) | argparse defaults to 2; JSON mode normalizes to contract |
+| Missing required arg | 2 (argparse default) | 1 (parse error envelope) | Same reason |
+| Session not found | 1 | 1 | Application-level error, same in both |
+| Command executed OK | 0 | 0 | Success path, identical |
+| Turn-loop timeout | 2 | 2 | Identical (#161 implementation) |
+
+**Practical rule for claws:** always pass `--output-format json`. This eliminates text-mode surprises and gives you the documented exit-code contract for every error path.
+
 ---
 
 ## One-Handler Pattern

tests/test_cross_channel_consistency.py

@@ -186,3 +186,57 @@ class TestCrossChannelConsistency:
             'Boolean fields must correlate with error block:\n' +
             '\n'.join(failures)
         )
+
+
+class TestTextVsJsonModeDivergence:
+    """Cycle #29: Document known text-mode vs JSON-mode exit code divergence.
+    
+    ERROR_HANDLING.md specifies the exit code contract applies ONLY when
+    --output-format json is set. Text mode follows argparse defaults (e.g.,
+    exit 2 for parse errors) while JSON mode normalizes to the contract
+    (exit 1 for parse errors).
+    
+    This test class LOCKS the expected divergence so:
+    1. Documentation stays aligned with implementation
+    2. Future changes to text mode behavior are caught as intentional
+    3. Claws consuming subprocess output can trust the docs
+    """
+
+    def test_unknown_command_text_mode_exits_2(self) -> None:
+        """Text mode: argparse default exit 2 for unknown subcommand."""
+        result = _run(['nonexistent-cmd'])
+        assert result.returncode == 2, (
+            f'text mode should exit 2 (argparse default), got {result.returncode}'
+        )
+
+    def test_unknown_command_json_mode_exits_1(self) -> None:
+        """JSON mode: normalized exit 1 for parse error (#178)."""
+        result = _run(['nonexistent-cmd', '--output-format', 'json'])
+        assert result.returncode == 1, (
+            f'JSON mode should exit 1 (protocol contract), got {result.returncode}'
+        )
+        envelope = json.loads(result.stdout)
+        assert envelope['error']['kind'] == 'parse'
+
+    def test_missing_required_arg_text_mode_exits_2(self) -> None:
+        """Text mode: argparse default exit 2 for missing required arg."""
+        result = _run(['exec-command'])  # missing name + prompt
+        assert result.returncode == 2, (
+            f'text mode should exit 2, got {result.returncode}'
+        )
+
+    def test_missing_required_arg_json_mode_exits_1(self) -> None:
+        """JSON mode: normalized exit 1 for parse error."""
+        result = _run(['exec-command', '--output-format', 'json'])
+        assert result.returncode == 1, (
+            f'JSON mode should exit 1, got {result.returncode}'
+        )
+
+    def test_success_path_identical_in_both_modes(self) -> None:
+        """Success exit codes are identical in both modes."""
+        text_result = _run(['list-sessions'])
+        json_result = _run(['list-sessions', '--output-format', 'json'])
+        assert text_result.returncode == json_result.returncode == 0, (
+            f'success exit should be 0 in both modes: '
+            f'text={text_result.returncode}, json={json_result.returncode}'
+        )