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.
- Fixed cmd_start() and cmd_continue() to wait for forked child
- Capture child PID with $! and wait before release_agent_slot
- This ensures slot is only released after child process completes
