The session-counting approach (PR #65) now properly handles agent
concurrency limits. Remove the broken slot-based mechanism:
- Remove acquire_agent_slot() and release_agent_slot() functions
- Remove AGENT_COUNT_FILE and AGENT_LOCK_FILE variables
- Remove unused run_with_limit() function
- Remove release-slot.sh script
- Update cmd_delegate to use fire-and-forget without slot management
Both cmd_start and cmd_delegate now use count_active_dev_sessions()
for concurrency checking.
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