feat(summary): revision-gated cache with stale-while-revalidate (STATE-WP-0066)

Replace the fixed 15s TTL on GET /state/summary with per-table revision
watermarks, stale-while-revalidate background refresh, and a progress-tail
section split. SQLAlchemy write hooks invalidate core or progress sections
on mutation. Adds tests, benchmark script, and operator docs.

mcp_server/TOOLS.md

@@ -89,7 +89,7 @@ entity while recording the missing progress event.
 | Tool | Key Args | When to use |
 |------|----------|-------------|
 | `get_domain_summary(domain_slug)` | `domain_slug`: e.g. `"railiance"` | **Domain session start.** Scoped snapshot: active workstreams, blocking decisions, last 5 events, repo SBOM status, compact capabilities list — ~10% of get_state_summary() token cost. |
-| `get_state_summary()` | — | **Cross-domain work / custodian sessions.** Full snapshot: totals, all blocking decisions, waiting tasks, all open workstreams, last 20 events. Large (~10k tokens). |
+| `get_state_summary()` | — | **Cross-domain work / custodian sessions.** Full snapshot: totals, all blocking decisions, waiting tasks, all open workstreams, last 20 events. Large (~10k tokens). API revision-caches unchanged snapshots (`X-StateHub-Cache: hit-revision`); use REST `?refresh=true` only when you need a forced rebuild. |
 | `get_topic(slug)` | `slug`: e.g. `"markitect"` | Deep-dive on one topic + its workstreams + recent events. |
 | `list_tasks(workstream_id, status?)` | `workstream_id`: UUID (required); `status?`: wait/todo/progress/done/cancel | List all tasks in a workstream. Use this to look up task UUIDs before calling `update_task_status`, or to verify which workplan tasks are already synced to the DB. |
 | `list_blocked_tasks(workstream_id?)` | optional filter | Legacy name: surfaces `wait` tasks, optionally scoped to one workstream. |