ROADMAP #80: session-lookup error copy lies about on-disk layout

Dogfooded 2026-04-17 on main HEAD 688295e against /tmp/claw-d4.
SessionStore::from_cwd at session_control.rs:32-40 places sessions
under .claw/sessions/<workspace_fingerprint>/ (16-char FNV-1a hex
at line 295-303), but format_no_managed_sessions and
format_missing_session_reference at line 516-526 advertise plain
.claw/sessions/ with no fingerprint context.

Concrete repro: fresh workspace, no sessions yet, .claw/sessions/
contains foo/ (hash dir, empty) + ffffffffffffffff/foreign.jsonl
(foreign workspace session). 'claw --resume latest' still says
'no managed sessions found in .claw/sessions/' even though that
directory is not empty -- the sessions just belong to other
workspace partitions.

Fix shape is ~30 lines: plumb the resolved sessions_root/workspace
into the two format helpers, optionally enumerate sibling partitions
so error copy tells the operator where sessions from other workspaces
are and why they're invisible.

Filed in response to Clawhip pinpoint nudge 1494615932222439456 in
#clawcode-building-in-public.

ROADMAP.md

@@ -1276,3 +1276,37 @@ Original filing (2026-04-13): user requested a `-acp` parameter to support ACP p
    **Blocker.** None. Scope is ~20 lines across `init.rs` (add `to_json_value` + `InitStatus::as_str`) and `main.rs` (switch `run_init` to hold the report and branch on format) plus one regression test.
 
    **Source.** Jobdori dogfood 2026-04-17 against `/tmp/init-test` and `/tmp/claw-clean` on main HEAD `9deaa29` in response to Clawhip pinpoint nudge at `1494608389068558386`. This is the mirror-image of ROADMAP #77 on the success side: the *shape* of success payloads is already structured for 7+ kinds, and `init` is the remaining odd-one-out that leaks structure only through prose.
+
+80. **Session-lookup error copy lies about where claw actually searches for managed sessions — omits the workspace-fingerprint namespacing** — dogfooded 2026-04-17 on main HEAD `688295e` against `/tmp/claw-d4`. Two session error messages advertise `.claw/sessions/` as the managed-session location, but the real on-disk layout (`rust/crates/runtime/src/session_control.rs:32-40` — `SessionStore::from_cwd`) places sessions under `.claw/sessions/<workspace_fingerprint>/` where `workspace_fingerprint()` at line 295-303 is a 16-char FNV-1a hex hash of the absolute CWD path. The gap is user-visible and trivially reproducible.
+
+   **Concrete repro on `/tmp/claw-d4` (fresh `git init` + first `claw ...` invocation auto-creates the hash dir).** After one `claw status` call, the disk layout looks like:
+   ```
+   .claw/sessions/
+   .claw/sessions/90ce0307fff7fef2/    <- workspace fingerprint dir, empty
+   ```
+   Then run `claw --output-format json --resume latest` and the error is:
+   ```
+   {"type":"error","error":"failed to restore session: no managed sessions found in .claw/sessions/\nStart `claw` to create a session, then rerun with `--resume latest`."}
+   ```
+   A claw that dumb-scans `.claw/sessions/` and sees the hash dir has no way to know: (a) what that hash dir is; (b) whether it is the "right" dir for the current workspace; (c) why the session it placed earlier at `.claw/sessions/s1/session.jsonl` is invisible; (d) why a foreign session at `.claw/sessions/ffffffffffffffff/foreign.jsonl` from a previous CWD is also invisible. The error copy as-written is a direct lie — `.claw/sessions/` contained two `.jsonl` files in my repro, and the error still said "no managed sessions found in `.claw/sessions/`".
+
+   **Contrast with the session-not-found error.** `format_missing_session_reference(reference)` at line 516-520 also advertises "managed sessions live in `.claw/sessions/`" — same lie. Both error strings were clearly written before the workspace-fingerprint partitioning shipped and were never updated when it landed; the fingerprint layout is commented in source (`session_control.rs:14-23`) as the intentional design so sessions from different CWDs don't collide, but neither the error messages nor `--help` nor `CLAUDE.md` expose that layout to the operator.
+
+   **Trace path.**
+    - `rust/crates/runtime/src/session_control.rs:32-40` — `SessionStore::from_cwd` computes `sessions_root = cwd.join(".claw").join("sessions").join(workspace_fingerprint(cwd))` and `fs::create_dir_all`s it.
+    - `rust/crates/runtime/src/session_control.rs:295-303` — `workspace_fingerprint()` returns the 16-char FNV-1a hex hash of `workspace_root.to_string_lossy()`.
+    - `rust/crates/runtime/src/session_control.rs:141-148` — `list_sessions()` scans `self.sessions_root` (i.e. the hashed dir) plus an optional legacy root — `.claw/sessions/` itself is never scanned as a flat directory.
+    - `rust/crates/runtime/src/session_control.rs:516-526` — the two `format_*` helpers that build the user-facing error copy hard-code `.claw/sessions/` with no workspace-fingerprint context and no `workspace_root` parameter plumbed in.
+
+   **Fix shape.**
+    - (a) Plumb the resolved `sessions_root` (or `workspace_root` + `workspace_fingerprint`) into the two error formatters so the error copy can point at the actual search path. Example: `no managed sessions found in .claw/sessions/90ce0307fff7fef2/ (workspace=/tmp/claw-d4)\nHint: claw partitions sessions per workspace fingerprint; sessions from other workspaces under .claw/sessions/ are intentionally invisible.\nStart `claw` in this workspace to create a session, then rerun with --resume latest.`
+    - (b) If `list_sessions()` scanned the hashed dir and found nothing but the parent `.claw/sessions/` contains *other* hash dirs with `.jsonl` content, surface that in the hint: "found N session(s) in other workspace partitions; none belong to the current workspace". This mirrors the information the user already sees on disk but never gets in the error.
+    - (c) Add a matching hint to `format_missing_session_reference` so `--resume <nonexistent-id>` also tells the truth about layout.
+    - (d) `CLAUDE.md`/README should document that `.claw/sessions/<hash>/` is intentional partitioning so operators tempted to symlink or merge directories understand why.
+    - (e) Unit coverage parallel to `workspace_fingerprint_is_deterministic_and_differs_per_path` at line 728+ — assert that `list_managed_sessions_for()` error text mentions the actual resolved fingerprint dir, not just `.claw/sessions/`.
+
+   **Acceptance.** A claw dumb-scanning `.claw/sessions/` and seeing non-empty content can tell from the error alone that the sessions belong to other workspace partitions and are intentionally invisible; error text points at the real search directory; and the workspace-fingerprint partitioning stops being surprise state hidden behind a misleading error string.
+
+   **Blocker.** None. Scope is ~30 lines across `session_control.rs:516-526` (re-shape the two helpers to accept the resolved path and optionally enumerate sibling partitions) plus the call sites that invoke them plus one unit test. No runtime behavior change; just error-copy accuracy + optional sibling-partition enumeration.
+
+   **Source.** Jobdori dogfood 2026-04-17 against `/tmp/claw-d4` on main HEAD `688295e` in response to Clawhip pinpoint nudge at `1494615932222439456`. Adjacent to ROADMAP #21 (`/session list` / resumed status contract) but distinct — this is the error-message accuracy gap, not the JSON-shape gap.