fix(cluster): stop cluster-apply crash-loops from the recovery-sidecar trap (#284)

* fix(cluster): stop cluster-apply crash-loops from the recovery-sidecar trap

A `cluster apply` carrying a schema change against a graph that has
non-main branches, or an unsupported "needs backfill" migration, armed a
recovery sidecar *before* calling the engine, then left it behind when the
engine rejected the apply pre-movement. The server refuses to boot while
any sidecar is pending, and re-running apply re-armed a fresh sidecar — an
unescapable crash loop. None of the engine rejections are bugs; the trap
is in the apply/serve choreography.

Three coordinated changes:

1. Preview before arming the sidecar. `cluster apply` now runs
   `preview_schema_apply_with_options` before `write_recovery_sidecar`, so
   parser/planner rejections (non-main branches, unsupported plan) fail
   loudly without leaving recovery work behind. The post-preview engine
   error path now deletes the sidecar when the live schema still matches
   the recorded digest (nothing moved), and keeps it only on real
   mid-movement failure — both branches covered by new engine-failpoint
   tests (cluster failpoints now enable omnigraph/failpoints).

2. Per-graph quarantine at serve time instead of whole-cluster refusal.
   A graph-attributed pending sidecar, an unopenable graph root, a query
   parse failure, or an unresolvable embedding provider now quarantines
   just that graph (logged loudly at every boot layer) while healthy
   graphs serve; `/graphs` lists only ready graphs and quarantined routes
   404. Cluster-global problems (missing/unreadable state, malformed or
   unattributable sidecars, shared-catalog or cluster-policy errors, zero
   healthy graphs) stay fail-fast. `--require-all-graphs` /
   OMNIGRAPH_REQUIRE_ALL_GRAPHS=1 restores all-or-nothing boot.

3. Backfill embedding-provider profile metadata on apply. Mirrors the
   existing policy-binding backfill: a pre-5A ledger missing
   `embedding_profile` is now detected as a metadata-only change and
   backfilled by a no-op apply, instead of bricking serve with
   `embedding_provider_profile_missing` forever.

Tests: trap (no sidecar after a rejected apply), both digest-cleanup
branches, per-graph quarantine (cluster + server), embedding backfill.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs: resilient cluster boot + recovery-sidecar trap fix

Amend RFC-005 D4 readiness posture (cluster-global fail-fast vs graph-local
quarantine; deviation #5 for --require-all-graphs), add the v0.7.0 release
note, and update the user cluster/server/deployment docs and the
OMNIGRAPH_REQUIRE_ALL_GRAPHS env var.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(cluster): surface sidecar-cleanup failures; document severity promotion

Address Greptile review on PR #284:

- The pre-movement sidecar cleanup fast-path discarded `delete_object`'s
  result, so a transient delete failure left the graph quarantined with no
  signal. Add `try_delete_object` (Result-returning) and emit a
  `recovery_sidecar_cleanup_failed` warning diagnostic on failure; the
  fire-and-forget `delete_object` now delegates to it.
- Document why the serve-time loop promotes every `list_recovery_sidecars`
  diagnostic to a cluster-fatal error (the listing only emits genuine
  read/parse/version failures, as warnings, whose blast radius serving
  cannot prove) and note the promote-by-code path if that ever changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/releases/v0.7.0.md

@@ -36,6 +36,12 @@ get faster and self-healing, and text embedding becomes provider-independent.
   single-graph flat-route mode, positional-`<URI>` boot, and `omnigraph.yaml`
   `graphs:`-map boot are gone — add or remove graphs with `cluster apply` and
   restart.
+- **Resilient cluster boot with strict opt-out.** Graph-attributed startup
+  failures now quarantine that graph and let healthy graphs serve; `/graphs`
+  lists only ready graphs, and quarantined graph routes return 404. Cluster-
+  global failures still refuse boot, and `--require-all-graphs` (or
+  `OMNIGRAPH_REQUIRE_ALL_GRAPHS=1`) restores fail-fast all-or-nothing startup
+  for operators who prefer any degraded graph to abort the process.
 - **One storage substrate + recovery liveness.** The cluster storage backend and
   the engine both go through one `StorageAdapter` (versioned read, conditional
   replace/CAS, prefix delete), exercised by a storage fault-injection matrix.