tandem up/down/status/logs for daemon management without systemd.
Control socket (Unix domain socket, JSON lines). Signal handling.
19 new tests (34 total). README restructured, systemd section removed.

Implements daemon management without systemd:
- tandem up: start background daemon, return immediately
- tandem down: stop daemon via control socket
- tandem status: health check with --json support
- tandem logs: stream logs from running daemon at any verbosity
- tandem serve: new flags (--log-level, --log-format, --control-socket, --daemon)

Control socket uses newline-delimited JSON over Unix domain socket.
Signal handling: SIGTERM/SIGINT for graceful shutdown, double-signal for immediate exit.

19 new tests across 4 test files (34 total, all passing).
Docs overhauled: README restructured with quickstart first, deployment sections updated.

- Rewrite AGENTS.md as clean execution guide (no version history)
- Remove v0→v1 migration language from all docs/
- Rename v1_slice2/v1_slice3 test files to slice2/slice3
- Update ARCHITECTURE.md, README.md, QA reports
- Clean up exec-plans, design-docs, product-specs
- All docs now describe current state only