feat: add worker CLI scaffold in Nim
Multi-agent coordination CLI with SQLite message bus:
- State machine: ASSIGNED -> WORKING -> IN_REVIEW -> APPROVED -> COMPLETED
- Commands: spawn, start, done, approve, merge, cancel, fail, heartbeat
- SQLite WAL mode, dedicated heartbeat thread, channel-based IPC
- cligen for CLI, tiny_sqlite for DB, ORC memory management
Design docs for branch-per-worker, state machine, message passing,
and human observability patterns.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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