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/user/clusters/index.md

@@ -221,7 +221,8 @@ applied revision is not safely servable. Each refusal names its remedy:
 | Boot error | Meaning | Remedy |
 |---|---|---|
 | `cluster_state_missing` | no ledger | `cluster import`, then `apply` |
-| `cluster_recovery_pending` | interrupted operation awaiting sweep | run `cluster apply` (or any state-mutating command), restart |
+| `cluster_recovery_pending` | graph was quarantined because an interrupted operation awaits sweep | run `cluster apply` (or any state-mutating command), restart |
+| `cluster_no_healthy_graphs` | every applied graph is quarantined or failed startup | sweep/fix the graph-specific failures, then restart |
 | `catalog_payload_missing` / `…_digest_mismatch` | catalog blob lost or tampered | `cluster refresh`, then `apply`, restart |
 | `policy_bindings_missing` | ledger predates binding metadata | re-run `cluster apply` (backfills), restart |
 | `cluster_empty` | applied revision has no graphs | apply a cluster with ≥1 graph |
@@ -231,6 +232,13 @@ A held *state lock* is deliberately **not** a boot error — the server reads
 the atomically-replaced ledger without locking, so serving never contends
 with an in-flight apply.
 
+When at least one graph is healthy, graph-attributed recovery sidecars and
+graph-local startup failures do not block the whole server. The affected
+graph is skipped, its graph-only policy bindings and queries are omitted,
+and `/graphs` lists only the ready graphs. Pass
+`omnigraph-server --require-all-graphs` or set
+`OMNIGRAPH_REQUIRE_ALL_GRAPHS=1` to make any such quarantine fail startup.
+
 ## 6. Deployment patterns
 
 - **Replicas**: any number of `--cluster` servers can serve the same config