feat: bridge directory metadata + stale-base preflight check

- Add CWD to SSE session events (kills Directory: unknown)
- Add stale-base preflight: verify HEAD matches expected base commit
- Warn on divergence before session starts

Author: code-yeongyu

rust/crates/runtime/src/lib.rs

@@ -38,6 +38,7 @@ mod session;
 #[cfg(test)]
 mod session_control;
 mod sse;
+pub mod stale_base;
 pub mod stale_branch;
 pub mod summary_compression;
 pub mod task_packet;
@@ -150,6 +151,10 @@ pub use session::{
     SessionFork, SessionPromptEntry,
 };
 pub use sse::{IncrementalSseParser, SseEvent};
+pub use stale_base::{
+    check_base_commit, format_stale_base_warning, read_claw_base_file, resolve_expected_base,
+    BaseCommitSource, BaseCommitState,
+};
 pub use stale_branch::{
     apply_policy, check_freshness, BranchFreshness, StaleBranchAction, StaleBranchEvent,
     StaleBranchPolicy,