feat: Matrix ChatOps bot (Task 5)

- Matrix bot via matrix-sdk: connect, join room, sync loop
- /fleet status: list all agents with status table
- /assign <agent> <issue>: manual task assignment
- /retry <issue>: re-queue failed/agent_lost task
- Notification formatting: task assigned/completed/failed, agent offline
- Per-agent thread support via Matrix Relation::Thread
- 15 tests: command parsing, notification formatting, fleet status table

Cargo.lock

@@ -53,6 +53,7 @@ dependencies = [
  "hmac",
  "matrix-sdk",
  "reqwest",
+ "ruma",
  "rusqlite",
  "serde",
  "serde_json",