feat: implement orchestrator core (Rust)

Task 1.1: ✅ Cargo.toml with axum, rusqlite, matrix-sdk, serde, etc.
Task 1.2: ✅ Directory structure: src/core, src/adapters, src/integrations, src/api
Task 1.5: ✅ config.example.toml with full schema
Task 2.1: ✅ Data models: Agent, Task, Receipt, Artifact, TaskEvent
Task 2.2: ✅ Event Store: SQLite append-only with task/agent tables
Task 2.3: ✅ Task state machine: created→assigned→running→completed/failed
Task 2.4: ✅ Global task queue with priority ordering
Task 2.5: ✅ Background timeout checker
Task 2.6: ✅ Retry policy with configurable max_retries

Compiles clean (warnings only, no errors).
API handler stubs in place for Phase 2.

config.example.toml

@@ -0,0 +1,21 @@
+[server]
+bind = "0.0.0.0"
+port = 9090
+
+[forgejo]
+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
+heartbeat_timeout_threshold = 3
+task_timeout_secs = 1800
+default_max_retries = 2