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.

Cargo.toml

@@ -2,7 +2,7 @@
 name = "agent-fleet"
 version = "0.1.0"
 edition = "2024"
-description = "Agent Fleet Platform - Multi-agent orchestration with Forgejo + Matrix"
+description = "Agent Fleet Platform - Multi-agent orchestration with Forgejo"
 license = "MIT"
 
 [dependencies]
@@ -22,13 +22,9 @@ rusqlite = { version = "0.32", features = ["bundled"] }
 # Configuration
 toml = "0.8"
 
-# HTTP client (for Forgejo API, Matrix API)
+# HTTP client (for Forgejo API)
 reqwest = { version = "0.12", features = ["json"] }
 
-# Matrix SDK
-matrix-sdk = "0.10"
-ruma = { version = "0.12", features = ["client-api-c", "rand", "unstable-msc3061", "unstable-msc2448"] }
-
 # Logging
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }