feat(execution): add SessionOrchestrator for autonomous session management

Implements fork/park/resume/complete lifecycle for topic-based sessions:
- SessionState: Active, Parked(reason), Completed(summary)
- SessionOrchestrator: register, park, resume, complete, fork, list, next_resumable
- SessionBudget: max_children, max_total_tokens, max_depth
- State transition history tracking
- Persists via session extension_data (no schema changes)
- 9 tests covering full lifecycle

Part of goose4-vu0h: Autonomous session management

Author: bioinfornatics

crates/goose/src/execution/mod.rs

@@ -5,6 +5,7 @@
 
 pub mod manager;
 pub mod pool;
+pub mod session_orchestrator;
 pub mod workspace;
 
 use serde::{Deserialize, Serialize};