feat: add session interface with channel-based local implementation
Session interface decouples UI from engine via channels:
- Send(input) starts agentic turn in background goroutine
- Events() returns channel for streaming events
- TurnResult() returns completed Turn after drain
- Cancel() propagates context cancellation
- Status() reports state, provider, model, token usage, turn count
Local implementation: engine runs on dedicated goroutine per turn,
events pushed to buffered channel (64), context cancellation
propagated. 5 tests.
vikingowl
c6b13f7cc8