fix: implement session-counting for MAX_CONCURRENT_AGENTS limit (fixes #63)

Replaced broken slot-based mechanism with session-counting:

- Added count_active_dev_sessions() function that counts actual
  session files in ~/.kugetsu/sessions/, excluding base.json and pm-agent.json
- Modified cmd_start() to check session count before creating new session:
  - If count >= MAX_CONCURRENT_AGENTS, reject with error
  - Otherwise allow new session creation
- Removed wait  since --fork returns immediately
- cmd_continue() no longer counts toward limit (existing sessions
  can always continue via --continue)

This properly enforces MAX_CONCURRENT_AGENTS while preserving --fork
functionality. The slot mechanism didn't work because opencode run
--fork returns immediately after forking, not after child completes.

skills/kugetsu/scripts/kugetsu

@@ -48,6 +48,21 @@ release_agent_slot() {
     ) 200>"$AGENT_LOCK_FILE"
 }
 
+count_active_dev_sessions() {
+    local count=0
+    if [ -d "$SESSIONS_DIR" ]; then
+        for session_file in "$SESSIONS_DIR"/*.json; do
+            if [ -f "$session_file" ]; then
+                local filename=$(basename "$session_file")
+                if [ "$filename" != "base.json" ] && [ "$filename" != "pm-agent.json" ]; then
+                    count=$((count + 1))
+                fi
+            fi
+        done
+    fi
+    echo "$count"
+}
+
 run_with_limit() {
     local log_file="$1"
     shift
@@ -852,19 +867,21 @@ cmd_start() {
     local before_set="${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
+    
+    # Session-counting: count actual dev sessions, reject if at limit
+    local active_count=$(count_active_dev_sessions)
+    if [ "$active_count" -ge "$MAX_CONCURRENT_AGENTS" ]; then
+        echo "Error: Max concurrent agents ($MAX_CONCURRENT_AGENTS) reached" >&2
+        echo "Active sessions: $active_count" >&2
         remove_worktree_for_issue "$issue_ref"
         exit 1
     fi
+    
     if [ "$DEBUG_MODE" = true ]; then
         opencode run "$message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1 | tee "$SESSIONS_DIR/$session_file.debug.log" &
     else
         opencode run "$message" --fork --session "$base_session_id" --dir "$worktree_path" 2>&1 &
     fi
-    local child_pid=$!
-    wait $child_pid
-    release_agent_slot
 
     local after_sessions=$(opencode session list 2>/dev/null | grep -oP '^ses_\w+' | sort)
     local new_session_id=""
@@ -933,10 +950,7 @@ cmd_continue() {
     local worktree_path=$(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
+    # Note: --continue always allowed (existing sessions don't count toward limit)
     if [ -n "$worktree_path" ] && [ -d "$worktree_path" ]; then
         echo "Using worktree: $worktree_path"
         if [ "$DEBUG_MODE" = true ]; then
@@ -951,9 +965,6 @@ cmd_continue() {
             opencode run "$message" --continue --session "$opencode_session_id" 2>&1 &
         fi
     fi
-    local child_pid=$!
-    wait $child_pid
-    release_agent_slot
 }
 
 cmd_list() {