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.

.gitignore

@@ -1 +1,6 @@
 .env.local
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+
+# Added by cargo
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+/target