Phase 4: shared-write hardening + group lenses

- vault_lock gains bounded retry/backoff (CHORUS_LOCK_RETRIES=4,
  CHORUS_LOCK_BACKOFF=0.5s); atomic_index_update and recall-index upkeep
  retry acquisition — unlocked fallback only after exhausted retries,
  loudly, so an already-written note is never stranded unindexed.
- Duplicate-gate candidates carry the existing note's author; STOP
  message says whose entity the capture collided with.
- recall --author <member> filters hits + linked context to one member's
  notes; recall briefs and --json surface the author: stamp.
- load --all-members appends a group-wide view (every member's scope +
  last-session heartbeat); roster parsing accepts folders as
  trailing-slash file entries (the REST API's actual shape).
- operating-contract.md concurrency section rewritten for N members.
- Two-member e2e suite committed as eval/test_multimember.py (26 checks
  incl. all four Phase-4 behaviors).
- Manifest -> 2.0.0-alpha.4; rebuilt chorus-memory.plugin.

Verified: 25/25 unit, scaffold, routing-sync, 5 mock e2e suites (incl.
new multimember), run_eval metrics unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -1,9 +1,9 @@
-# chorus-memory — v2.0.0-alpha.3
+# chorus-memory — v2.0.0-alpha.4
 
 > **CHORUS is a group-based fork of [ECHO](https://git.alwisp.com/jason/echo)** — one shared
 > Obsidian vault serving a group of members instead of a single operator. The conversion
 > plan and settled design decisions live in [CHORUS-PLAN.md](CHORUS-PLAN.md). The pre-fork
-> ECHO v1.5.1 state is preserved at the `echo-fork-point` tag. **Phases 1–3 (through per-member
+> ECHO v1.5.1 state is preserved at the `echo-fork-point` tag. **Phases 1–4 (through shared-write
 > namespacing) are done**; the body of this README below the fold still describes the
 > inherited single-operator behavior and is rewritten in Phase 5.
 > Deployment endpoint: `https://chorusapi.mpm.to`. CHORUS is a **clean break** from