feat(kugetsu): add lock mechanism for worktree coordination

Add lock mechanism to prevent concurrent access to worktrees:
- Add LOCKS_DIR constant (~/.kugetsu/locks)
- Add acquire_lock() - acquires lock file with PID, session_id, timestamp
- Add release_lock() - releases lock if held by current process
- Add release_all_locks() - releases all locks for a session
- Add check_lock() - check if issue is locked
- Modify cmd_start to acquire lock before creating worktree
- Modify cmd_destroy to release lock when destroying session
- Add trap for cleanup on EXIT/INT/TERM

Lock files named after issue ref. Stale lock detection: if PID
no longer exists, lock is considered stale.

Fixes #71

skills/kugetsu/scripts/kugetsu

@@ -9,6 +9,7 @@ INDEX_FILE="$KUGETSU_DIR/index.json"
 NOTIFICATIONS_FILE="$KUGETSU_DIR/notifications.json"
 LOGS_DIR="$KUGETSU_DIR/logs"
 ENV_DIR="${ENV_DIR:-$KUGETSU_DIR/env}"
+LOCKS_DIR="$KUGETSU_DIR/locks"
 MAX_CONCURRENT_AGENTS="${MAX_CONCURRENT_AGENTS:-3}"
 
 # Load user config overrides (~/.kugetsu/config)
@@ -56,6 +57,87 @@ count_active_dev_sessions() {
     echo "$count"
 }
 
+acquire_lock() {
+    local issue_ref="$1"
+    local session_id="$2"
+    local lock_file="$LOCKS_DIR/${issue_ref//[^a-zA-Z0-9._-]/_}.lock"
+    
+    mkdir -p "$LOCKS_DIR"
+    
+    if [ -f "$lock_file" ]; then
+        local lock_pid=$(cat "$lock_file" 2>/dev/null | cut -d: -f1)
+        local lock_session=$(cat "$lock_file" 2>/dev/null | cut -d: -f2)
+        
+        if [ -n "$lock_pid" ] && ! kill -0 "$lock_pid" 2>/dev/null; then
+            echo "Stale lock detected, removing..."
+            rm -f "$lock_file"
+        elif [ "$lock_session" = "$session_id" ]; then
+            echo "Already holding lock for $issue_ref"
+            return 0
+        else
+            echo "Error: $issue_ref is locked by session $lock_session (PID $lock_pid)" >&2
+            return 1
+        fi
+    fi
+    
+    echo "${BASHPID}:${session_id}:$(date +%s)" > "$lock_file"
+    echo "Lock acquired for $issue_ref: $lock_file"
+    return 0
+}
+
+release_lock() {
+    local issue_ref="$1"
+    local lock_file="$LOCKS_DIR/${issue_ref//[^a-zA-Z0-9._-]/_}.lock"
+    
+    if [ ! -f "$lock_file" ]; then
+        return 0
+    fi
+    
+    local lock_pid=$(cat "$lock_file" 2>/dev/null | cut -d: -f1)
+    
+    if [ "$lock_pid" = "$BASHPID" ]; then
+        rm -f "$lock_file"
+        echo "Lock released for $issue_ref"
+    else
+        echo "Error: Cannot release lock held by PID $lock_pid (current: $BASHPID)" >&2
+        return 1
+    fi
+}
+
+release_all_locks() {
+    local session_id="$1"
+    if [ ! -d "$LOCKS_DIR" ]; then
+        return 0
+    fi
+    for lock_file in "$LOCKS_DIR"/*.lock; do
+        [ -f "$lock_file" ] || continue
+        local lock_session=$(cat "$lock_file" 2>/dev/null | cut -d: -f2)
+        if [ "$lock_session" = "$session_id" ]; then
+            rm -f "$lock_file"
+            echo "Released stale lock: $(basename "$lock_file")"
+        fi
+    done
+}
+
+check_lock() {
+    local issue_ref="$1"
+    local lock_file="$LOCKS_DIR/${issue_ref//[^a-zA-Z0-9._-]/_}.lock"
+    
+    if [ -f "$lock_file" ]; then
+        local lock_pid=$(cat "$lock_file" 2>/dev/null | cut -d: -f1)
+        local lock_session=$(cat "$lock_file" 2>/dev/null | cut -d: -f2)
+        
+        if [ -n "$lock_pid" ] && ! kill -0 "$lock_pid" 2>/dev/null; then
+            echo "Stale lock detected, removing..."
+            rm -f "$lock_file"
+            return 1
+        fi
+        echo "Locked by session $lock_session (PID $lock_pid)"
+        return 0
+    fi
+    return 1
+}
+
 usage() {
     cat << 'EOF'
 kugetsu - OpenCode Session Manager (Issue-Driven)
@@ -1097,6 +1179,14 @@ cmd_start() {
     fi
 
     local worktree_path=$(issue_ref_to_worktree_path "$issue_ref")
+    
+    trap 'release_lock "$issue_ref" 2>/dev/null; exit' EXIT INT TERM
+    
+    if ! acquire_lock "$issue_ref" "$base_session_id"; then
+        echo "Error: Could not acquire lock for '$issue_ref'" >&2
+        exit 1
+    fi
+    
     create_worktree "$issue_ref"
 
     local session_file="$(issue_ref_to_filename "$issue_ref").json"
@@ -1163,6 +1253,8 @@ cmd_start() {
 
     echo "Session started for '$issue_ref': $new_session_id"
     echo "Worktree: $worktree_path"
+    
+    release_lock "$issue_ref"
 }
 
 cmd_continue() {
@@ -1416,6 +1508,7 @@ cmd_destroy() {
 
     if [ "$force" = true ]; then
         remove_worktree_for_issue "$target"
+        release_lock "$target" 2>/dev/null || true
         rm -f "$session_path"
         remove_issue_from_index "$target"
         echo "Session for '$target' destroyed"
@@ -1425,6 +1518,7 @@ cmd_destroy() {
         read reply
         if [ "$reply" = "y" ] || [ "$reply" = "Y" ]; then
             remove_worktree_for_issue "$target"
+            release_lock "$target" 2>/dev/null || true
             rm -f "$session_path"
             remove_issue_from_index "$target"
             echo "Session for '$target' destroyed"