Add concurrent agent limiting to kugetsu CLI

- Add MAX_CONCURRENT_AGENTS (default: 3) to limit concurrent agents
- Implement acquire_agent_slot() and release_agent_slot() with flock
- Wrap cmd_start, cmd_continue, and cmd_delegate with slot management
- cmd_delegate holds slot until background task completes (fire-and-forget + blocking)
- Add basic concurrency tests to test suite

skills/kugetsu/scripts/kugetsu

@@ -8,6 +8,56 @@ REPOS_CONFIG="$KUGETSU_DIR/repos.json"
 INDEX_FILE="$KUGETSU_DIR/index.json"
 NOTIFICATIONS_FILE="$KUGETSU_DIR/notifications.json"
 LOGS_DIR="$KUGETSU_DIR/logs"
+MAX_CONCURRENT_AGENTS="${MAX_CONCURRENT_AGENTS:-3}"
+AGENT_COUNT_FILE="$KUGETSU_DIR/.agent_count"
+AGENT_LOCK_FILE="$KUGETSU_DIR/.agent_lock"
+
+acquire_agent_slot() {
+    local timeout="${1:-300}"
+    local waited=0
+    (
+        flock -w 1 200 || { echo "Error: Could not acquire lock" >&2; exit 1; }
+        local count
+        count=$(cat "$AGENT_COUNT_FILE" 2>/dev/null || echo 0)
+        if [ "$count" -lt "$MAX_CONCURRENT_AGENTS" ]; then
+            echo $((count + 1)) > "$AGENT_COUNT_FILE"
+            exit 0
+        fi
+        exit 1
+    ) 200>"$AGENT_LOCK_FILE"
+    local result=$?
+    if [ $result -ne 0 ]; then
+        local count
+        count=$(cat "$AGENT_COUNT_FILE" 2>/dev/null || echo 0)
+        if [ $waited -ge $timeout ]; then
+            echo "Error: Timeout waiting for agent slot (max: $MAX_CONCURRENT_AGENTS, current: $count)" >&2
+        fi
+        return 1
+    fi
+    return 0
+}
+
+release_agent_slot() {
+    (
+        flock -w 1 200 || true
+        local count
+        count=$(cat "$AGENT_COUNT_FILE" 2>/dev/null || echo 0)
+        if [ "$count" -gt 0 ]; then
+            echo $((count - 1)) > "$AGENT_COUNT_FILE"
+        fi
+    ) 200>"$AGENT_LOCK_FILE"
+}
+
+run_with_limit() {
+    local log_file="$1"
+    shift
+    local cmd=("$@")
+    (
+        "${cmd[@]}" >> "$log_file" 2>&1
+        release_agent_slot
+    ) &
+    disown
+}
 
 usage() {
     cat << 'EOF'
@@ -83,6 +133,7 @@ EOF
 
 ensure_dirs() {
     mkdir -p "$SESSIONS_DIR"
+    [ -f "$AGENT_COUNT_FILE" ] || echo 0 > "$AGENT_COUNT_FILE"
 }
 
 ensure_worktree_dir() {
@@ -502,7 +553,11 @@ cmd_delegate() {
     
     mkdir -p "$LOGS_DIR"
     local log_file="$LOGS_DIR/delegate-$(date +%s).log"
-    nohup opencode run --continue --session "$pm_session" "$message" > "$log_file" 2>&1 &
+    if ! acquire_agent_slot; then
+        echo "Error: Max concurrent agents ($MAX_CONCURRENT_AGENTS) reached. Try again later." >&2
+        exit 1
+    fi
+    nohup sh -c "opencode run --continue --session '$pm_session' '$message' >> '$log_file' 2>&1; release_agent_slot" > /dev/null 2>&1 &
     disown
     echo "Delegated to PM agent (logged to $(basename "$log_file"))"
 }
@@ -796,11 +851,19 @@ cmd_start() {
     local before_set="${before_sessions//$'\n'/|}"
 
     echo "Forking session for '$issue_ref'..."
+    if ! acquire_agent_slot; then
+        echo "Error: Max concurrent agents ($MAX_CONCURRENT_AGENTS) reached. Try again later." >&2
+        remove_worktree_for_issue "$issue_ref"
+        exit 1
+    fi
+    trap release_agent_slot EXIT
     if [ "$DEBUG_MODE" = true ]; then
         opencode run --fork --session "$base_session_id" "$message" --workdir "$worktree_path" 2>&1 | tee "$SESSIONS_DIR/$session_file.debug.log"
     else
         opencode run --fork --session "$base_session_id" "$message" --workdir "$worktree_path" 2>&1
     fi
+    release_agent_slot
+    trap - EXIT
 
     local after_sessions=$(opencode session list 2>/dev/null | grep -oP '^ses_\w+' | sort)
     local new_session_id=""
@@ -869,6 +932,11 @@ cmd_continue() {
     local worktree_path=$(python3 -c "import json; print(json.load(open('$session_path')).get('worktree_path', ''))" 2>/dev/null || echo "")
 
     echo "Continuing session for '$session_name'..."
+    if ! acquire_agent_slot; then
+        echo "Error: Max concurrent agents ($MAX_CONCURRENT_AGENTS) reached. Try again later." >&2
+        exit 1
+    fi
+    trap release_agent_slot EXIT
     if [ -n "$worktree_path" ] && [ -d "$worktree_path" ]; then
         echo "Using worktree: $worktree_path"
         if [ "$DEBUG_MODE" = true ]; then
@@ -883,6 +951,8 @@ cmd_continue() {
             opencode run --continue --session "$opencode_session_id" "$message"
         fi
     fi
+    release_agent_slot
+    trap - EXIT
 }
 
 cmd_list() {