huskies: merge 1199 story Orphaned build-dir GC: reclaim dead worktree targets without touching warm caches

server/src/agents/merge/mod.rs

@@ -4,7 +4,7 @@ use serde::{Deserialize, Serialize};
 
 mod squash;
 
-pub(crate) use squash::run_squash_merge;
+pub(crate) use squash::{merge_lock_is_free, run_squash_merge};
 
 /// Typed outcome of a completed squash-merge operation.
 ///

server/src/agents/merge/squash/mod.rs

@@ -17,6 +17,19 @@ use crate::config::ProjectConfig;
 /// causing `git cherry-pick merge-queue/…` to fail with "bad revision".
 static MERGE_LOCK: Mutex<()> = Mutex::new(());
 
+/// Returns `true` when no squash-merge is currently running, i.e. the merge
+/// lock is free.
+///
+/// Used by the build-directory GC pass (story 1199) to decide whether
+/// `.huskies/merge_workspace` is safe to reclaim. This is a best-effort,
+/// momentary check — the lock is not held across the reclaim itself, so a
+/// merge that starts immediately afterward can still race with GC. That's
+/// acceptable: the GC pass tolerates races and skips on error rather than
+/// failing the whole pass.
+pub(crate) fn merge_lock_is_free() -> bool {
+    MERGE_LOCK.try_lock().is_ok()
+}
+
 /// Resolve the base branch for `project_root` from config, or auto-detect it.
 fn resolve_base_branch(project_root: &Path) -> String {
     let configured = crate::config::ProjectConfig::load(project_root)