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.

src/core/event_store.rs

@@ -275,6 +275,36 @@ impl EventStore {
             .collect::<SqlResult<Vec<_>>>()
     }
 
+    pub fn list_tasks(
+        &self,
+        status: Option<&str>,
+        agent_id: Option<&str>,
+    ) -> SqlResult<Vec<Task>> {
+        let mut sql = String::from(
+            "SELECT task_id, source, task_type, priority, status, assigned_agent_id,
+                    requirements, labels, created_at, assigned_at, started_at, completed_at,
+                    retry_count, max_retries, timeout_seconds
+             FROM tasks WHERE 1=1",
+        );
+        let mut param_values: Vec<Box<dyn rusqlite::types::ToSql>> = Vec::new();
+
+        if let Some(s) = status {
+            sql.push_str(" AND status = ?");
+            param_values.push(Box::new(s.to_string()));
+        }
+        if let Some(a) = agent_id {
+            sql.push_str(" AND assigned_agent_id = ?");
+            param_values.push(Box::new(a.to_string()));
+        }
+        sql.push_str(" ORDER BY created_at DESC");
+
+        let params: Vec<&dyn rusqlite::types::ToSql> = param_values.iter().map(|p| p.as_ref()).collect();
+
+        let mut stmt = self.conn.prepare(&sql)?;
+        stmt.query_map(params.as_slice(), Self::row_to_task)?
+            .collect::<SqlResult<Vec<_>>>()
+    }
+
     // ─── Task/event write operations ─────────────────────────────
 
     pub fn insert_task(&self, task: &Task) -> SqlResult<()> {