feat(matrix-bridge-dagi): M4–M11 + soak infrastructure (debug inject endpoint)

Includes all milestones M4 through M11:
- M4: agent discovery (!agents / !status)
- M5: node-aware routing + per-node observability
- M6: dynamic policy store (node/agent overrides, import/export)
- M7: Prometheus alerts + Grafana dashboard + metrics contract
- M8: node health tracker + soft failover + sticky cache + HA persistence
- M9: two-step confirm + diff preview for dangerous commands
- M10: auto-backup, restore, retention, policy history + change detail
- M11: soak scenarios (CI tests) + live soak script

Soak infrastructure (this commit):
- POST /v1/debug/inject_event (guarded by DEBUG_INJECT_ENABLED=false)
- _preflight_inject() and _check_wal() in soak script
- --db-path arg for WAL delta reporting
- Runbook sections 2a/2b/2c: Step 0 and Step 1 exact commands

Made-with: Cursor

services/matrix-bridge-dagi/app/mixed_routing.py

@@ -309,3 +309,25 @@ def reply_prefix(agent_id: str, is_mixed: bool) -> str:
         return ""
     # Capitalise first letter of agent name: "sofiia" → "Sofiia"
     return f"{agent_id.capitalize()}: "
+
+
+def build_override_config(
+    base_config: MixedRoomConfig,
+    room_id: str,
+    agents: List[str],
+    default_agent: str,
+) -> MixedRoomConfig:
+    """
+    M6.1: Build a temporary MixedRoomConfig that uses a dynamic store override
+    for room_id while keeping all other rooms from base_config unchanged.
+
+    Used in _enqueue_from_mixed_room to inject PolicyStore agent overrides
+    without mutating the shared base configuration.
+    """
+    rooms = dict(base_config.rooms)
+    rooms[room_id] = MixedRoom(
+        room_id=room_id,
+        agents=agents,
+        default_agent=default_agent,
+    )
+    return MixedRoomConfig(rooms=rooms)