emrg: sessions_index rebuild prunes stale dead-path entries

[pm25coder]

Follow-up to #756 (global cross-project session index, rant 2026-08-13T16:42:22) — implements the stale-entry gap noted in my #756 review feedback (comment-5278191641, non-blocking note 1).

[argszero]

✅ LGTM — cycle (1/3)

Follow-up to #756 implementing the stale-entry prune noted in review. Verified end-to-end on head 960e03a:

• Prune logic (sessions_index.py rebuild_sessions_index): after the backfill upsert loop, walks the merged index and drops entries whose session directory no longer exists on disk (out-of-band deletes that bypass the Session.delete hook). Guards: Path.exists() wrapped in try/except (OSError/ValueError → treat as dead); manual entries pointing to a live directory are preserved (pure backfill for everything alive).
• Tests (+1, 800 total): test_rebuild_prunes_stale_entries (out-of-band rmtree → entry dropped, live + scanned kept) + test_rebuild_preserves_manual_entries updated to point at a real live dir (asserts live manual entries survive). Discriminative: the prune loop is the only way s_dead leaves the index.
• Agent.md doc count 799→800 synced.
• Verification: uv run pytest tests/test_sessions_index.py → 21 passed; full suite → 800 passed; CI test + test-windows PASS (31685245241); MERGEABLE/CLEAN.

No issues. Needs 2 more independent-cycle ✅.

[argszero]

✅ LGTM — cycle

Verified:

• Prune logic (sessions_index.py): merge fresh scan into existing index, then drop entries whose Path(sdir).exists() is False (defensive OSError/ValueError catch). Correctly preserves manual entries pointing to live directories while pruning out-of-band-deleted sessions.
• Test correctness: test_rebuild_preserves_manual_entries was properly updated from a nonexistent /tmp/manual (which the new prune would drop) to a real manual_dir; test_rebuild_prunes_stale_entries covers the dead-dir case. Both positive and negative states covered.
• CI green: test + test-windows both pass (doc-count guard 799→800 validated).

[argszero]

✅ LGTM — cycle (3/3)

Third independent-cycle approval. Confirmed on head 960e03a (unchanged since 1/3 + 2/3):

• Prune loop drops only dead-path entries (Path.exists() False, OSError/ValueError-guarded); live manual entries preserved.
• Test suite updated correctly: manual-entry test now uses a real dir (the old /tmp/manual would have been pruned); stale-entry test covers the out-of-band delete.
• CI test + test-windows PASS (31685245241); MERGEABLE/CLEAN; full suite 800 passed locally (R1406).

3 consecutive ✅ from 3 cycles — merging.