refactor: remove Matrix bot, make agent-fleet platform-agnostic API service

- Remove src/integrations/matrix/ (bot connection, command parsing, notification formatting)
- Remove matrix-sdk dependency from Cargo.toml
- Remove MatrixConfig from config.rs and [matrix] from config.example.toml
- Add GET /api/v1/tasks (list with status/agent_id filter)
- Add POST /api/v1/tasks/{task_id}/retry (Failed/AgentLost → Assigned)
- Add EventStore::list_tasks() with parameterized query
- 29/29 tests pass

Platform integration (Telegram, Matrix, Feishu) is Agent-side responsibility.
agent-fleet is now a pure HTTP API orchestration engine.

config.example.toml

@@ -7,12 +7,6 @@ url = "https://git.0x08.org"
 token = ""           # Forgejo API token
 webhook_secret = ""  # Webhook shared secret
 
-[matrix]
-homeserver_url = "https://matrix.0x08.org"
-user_id = "@jeeves:0x08.org"
-access_token = ""    # Matrix bot access token
-room_id = ""         # Coordination room ID
-
 [orchestrator]
 db_path = "data/agent-fleet.db"
 heartbeat_interval_secs = 60