feat(cli): add lifecycle subcommands

[kgskr]

Summary

• add serve, start, status, and shutdown CLI lifecycle commands while keeping bare codex-lb-cinamon invocation as foreground serve
• add background runtime helpers for PID metadata, readiness checks, and graceful shutdown
• document macOS/Linux and Windows usage and add OpenSpec artifacts for the new CLI lifecycle capability

Verification

• .venv/bin/python -m pytest tests/unit/test_cli.py tests/unit/test_cli_runtime.py -q
• .venv/bin/python -m ruff check app/cli.py app/cli_runtime.py tests/unit/test_cli.py tests/unit/test_cli_runtime.py
• manual smoke: start -> status -> /health/live -> shutdown with temporary pid/log/db files