feat(controller): expose instance-level fleet identity and health snapshots

Add a pure fleet snapshot assessor and a read-only controller/reconciler
MCP tool so multi-instance fleets can be enumerated by client_instance_id
without treating shared client_type as duplication. Trusted launcher
instance IDs, classification (missing/unmanifested/collisions/historical),
registry schema extensions, docs, and regression tests for #978.

Closes #978

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

task_capability_map.py

@@ -163,6 +163,18 @@ TASK_CAPABILITY_MAP: dict[str, dict[str, str]] = {
         "permission": "gitea.read",
         "role": "author",
     },
+    # #978: instance-level fleet identity/health snapshot. gitea.read is the
+    # operation gate; the tool additionally restricts role_kind to
+    # controller|reconciler so author/reviewer/merger cannot use it as a
+    # mutation surface and no unrelated write permission is introduced.
+    "snapshot_instance_fleet": {
+        "permission": "gitea.read",
+        "role": "controller",
+    },
+    "gitea_snapshot_instance_fleet": {
+        "permission": "gitea.read",
+        "role": "controller",
+    },
     # #644: Phase 2 Web Console recovery tasks.
     "clear_stale_binding": {
         "permission": "gitea.read",