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

@@ -9,7 +9,10 @@ INDEX_FILE="$KUGETSU_DIR/index.json"
 NOTIFICATIONS_FILE="$KUGETSU_DIR/notifications.json"
 LOGS_DIR="$KUGETSU_DIR/logs"
 ENV_DIR="${ENV_DIR:-$KUGETSU_DIR/env}"
+QUEUE_FILE="$KUGETSU_DIR/queue.json"
+LOCKS_DIR="$KUGETSU_DIR/locks"
 MAX_CONCURRENT_AGENTS="${MAX_CONCURRENT_AGENTS:-3}"
+POLL_INTERVAL="${POLL_INTERVAL:-600}"
 
 # Load user config overrides (~/.kugetsu/config)
 if [ -f "$KUGETSU_DIR/config" ]; then
@@ -56,6 +59,87 @@ count_active_dev_sessions() {
     echo "$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)
@@ -595,6 +679,116 @@ cmd_logs() {
     done
 }
 
+init_queue() {
+    if [ ! -f "$QUEUE_FILE" ]; then
+        cat > "$QUEUE_FILE" << 'EOF'
+{
+  "dev_followups": [],
+  "user_interrupts": [],
+  "background": []
+}
+EOF
+    fi
+}
+
+cmd_queue() {
+    local action="${1:-}"
+    
+    init_queue
+    
+    case "$action" in
+        ""|"list")
+            local total=0
+            echo "Queue status:"
+            for tier in dev_followups user_interrupts background; do
+                local count=$(python3 -c "import json; d=json.load(open('$QUEUE_FILE')); print(len(d.get('$tier', [])))" 2>/dev/null || echo 0)
+                total=$((total + count))
+                if [ "$count" -eq 0 ]; then
+                    echo "  $tier (0): (empty)"
+                else
+                    echo "  $tier ($count):"
+                    python3 -c "import json, sys; d=json.load(open('$QUEUE_FILE')); [print(f'    [{t[\"id\"]}] {t[\"message\"][:60]}') for t in d.get('$tier', [])]" 2>/dev/null || echo "    (error reading)"
+                fi
+            done
+            echo "Total queued: $total"
+            ;;
+        "enqueue")
+            local tier="${2:-}"
+            local message="${3:-}"
+            if [ -z "$tier" ] || [ -z "$message" ]; then
+                echo "Usage: kugetsu queue enqueue <tier> <message>" >&2
+                echo "  tier: dev_followups, user_interrupts, or background" >&2
+                exit 1
+            fi
+            if [[ ! "$tier" =~ ^(dev_followups|user_interrupts|background)$ ]]; then
+                echo "Error: Invalid tier '$tier'" >&2
+                echo "  Valid tiers: dev_followups, user_interrupts, background" >&2
+                exit 1
+            fi
+            local id="qe-$(date +%s)-$$"
+            python3 << EOF
+import json
+with open('$QUEUE_FILE', 'r') as f:
+    d = json.load(f)
+d.setdefault('$tier', []).append({
+    'id': '$id',
+    'message': '$message',
+    'created': '$(date -Iseconds)'
+})
+with open('$QUEUE_FILE', 'w') as f:
+    json.dump(d, f, indent=2)
+print('Enqueued to $tier: [$id] $message')
+EOF
+            ;;
+        "dequeue")
+            local tier="${2:-}"
+            local result=$(python3 << EOF
+import json
+with open('$QUEUE_FILE', 'r') as f:
+    d = json.load(f)
+tiers = ['dev_followups', 'user_interrupts', 'background'] if not '$tier' else ['$tier']
+for t in tiers:
+    if d.get(t) and len(d[t]) > 0:
+        task = d[t].pop(0)
+        with open('$QUEUE_FILE', 'w') as f:
+            json.dump(d, f, indent=2)
+        print(f'{t}|{task["id"]}|{task["message"]}')
+        break
+else:
+    print('Queue empty')
+EOF
+)
+            if [ "$result" = "Queue empty" ]; then
+                echo "$result"
+                exit 1
+            fi
+            echo "$result"
+            ;;
+        "clear")
+            cat > "$QUEUE_FILE" << 'EOF'
+{
+  "dev_followups": [],
+  "user_interrupts": [],
+  "background": []
+}
+EOF
+            echo "Queue cleared"
+            ;;
+        *)
+            echo "Usage: kugetsu queue <list|enqueue|dequeue|clear>" >&2
+            echo "" >&2
+            echo "Commands:" >&2
+            echo "  list                    Show queue status" >&2
+            echo "  enqueue <tier> <msg>  Add task to queue" >&2
+            echo "  dequeue [tier]         Remove and return next task" >&2
+            echo "  clear                   Clear all queued tasks" >&2
+            echo "" >&2
+            echo "Tiers: dev_followups, user_interrupts, background" >&2
+            exit 1
+            ;;
+    esac
+}
+
 cmd_env() {
     local action="${1:-}"
     local agent_type="${2:-}"
@@ -1151,6 +1345,14 @@ 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"
@@ -1217,6 +1419,8 @@ cmd_start() {
 
     echo "Session started for '$issue_ref': $new_session_id"
     echo "Worktree: $worktree_path"
+    
+    release_lock "$issue_ref"
 }
 
 cmd_continue() {
@@ -1470,6 +1674,7 @@ 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"
@@ -1479,6 +1684,7 @@ 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 '$target' destroyed"
@@ -1523,6 +1729,9 @@ main() {
         server)
             cmd_server "$@"
             ;;
+        queue)
+            cmd_queue "$@"
+            ;;
         env)
             cmd_env "$@"
             ;;