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>

chorus-memory.plugin.src/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "chorus-memory",
-  "version": "2.0.0-alpha.3",
+  "version": "2.0.0-alpha.4",
   "description": "Group-based persistent memory via the shared CHORUS Obsidian vault over the Obsidian Local REST API. Cross-platform Python client: one-call capture/resolve/recall/link/triage over an entity index, hybrid BM25 + graph recall spanning entities + sessions/journal (recency/status-aware), a pre-write duplicate gate, complete-frontmatter capture, session hooks that self-fire load/reflect, offline write-ahead queue, lock-guarded concurrency, linter-enforced routing, and /chorus-* commands.",
   "author": {
     "name": "Jason Stedwell"