feat: harden issue lock store against concurrent session clobbering (Closes #438)

Replace read-modify-write on the global /tmp/gitea_issue_lock.json pointer with
atomic per-issue locks under GITEA_ISSUE_LOCK_DIR using flock serialization,
PID/session metadata, fail-closed stale recovery, and pre-mutation freshness
re-checks in gitea_create_pr. Queue inventory now surfaces live_issue_locks.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

scripts/worktree-start

@@ -37,14 +37,27 @@ fi
 
 branch="$1"
 start_ref="${2:-prgs/master}"
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+repo_root="$(cd "$script_dir/.." && pwd)"
 
 # Enforce issue-linked, traceable branch names (issue → branch → worktree → PR).
 if [[ "$allow_unlinked" -eq 0 ]]; then
-  if [[ ! -f "/tmp/gitea_issue_lock.json" ]]; then
-    echo "Error: Issue lock file '/tmp/gitea_issue_lock.json' is missing. You must lock exactly one issue before branch creation (fail closed)." >&2
+  locked_branch=$(PYTHONPATH="$repo_root" python3 - <<'PY' "$branch")
+import sys
+import issue_lock_store
+
+branch = sys.argv[1]
+lock = issue_lock_store.resolve_lock_for_branch(branch)
+if not lock:
+    print("", end="")
+    sys.exit(1)
+print(lock.get("branch_name", ""), end="")
+PY
+  )
+  if [[ -z "$locked_branch" ]]; then
+    echo "Error: No issue lock found for branch '$branch'. Lock the issue before branch creation (fail closed)." >&2
     exit 2
   fi
-  locked_branch=$(python3 -c "import json; print(json.load(open('/tmp/gitea_issue_lock.json')).get('branch_name', ''))")
   if [[ "$branch" != "$locked_branch" ]]; then
     echo "Error: Requested branch '$branch' does not match locked branch '$locked_branch' (fail closed)." >&2
     exit 2
@@ -68,8 +81,6 @@ EOF
   fi
 fi
 
-script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
-repo_root="$(cd "$script_dir/.." && pwd)"
 worktree_name="${branch//\//-}"
 worktree_path="$repo_root/branches/$worktree_name"