feat(runtime): stale-branch detection with freshness check and policy

rust/crates/runtime/src/lib.rs

@@ -20,6 +20,7 @@ mod remote;
 pub mod sandbox;
 mod session;
 mod sse;
+mod stale_branch;
 pub mod task_registry;
 pub mod team_cron_registry;
 mod usage;
@@ -99,6 +100,10 @@ pub use session::{
     ContentBlock, ConversationMessage, MessageRole, Session, SessionCompaction, SessionError,
     SessionFork,
 };
+pub use stale_branch::{
+    apply_policy, check_freshness, BranchFreshness, StaleBranchAction, StaleBranchEvent,
+    StaleBranchPolicy,
+};
 pub use sse::{IncrementalSseParser, SseEvent};
 pub use worker_boot::{
     Worker, WorkerEvent, WorkerEventKind, WorkerFailure, WorkerFailureKind, WorkerReadySnapshot,