[codex] Add Tether Agent service installer

[rylinjames]

Summary

• add systemd user service and launchd plist rendering for Tether Agent autostart
• add tether agent install-service and uninstall-service
• keep both commands dry-run by default; --apply writes or removes the service file

Validation

• PYTHONPATH=src pytest tests/test_agent_*.py -q
• PYTHONPATH=src ruff check src/tether/agent tests/test_agent_*.py
• PYTHONPATH=src ruff check src/tether/cli.py --select E9,F63,F7,F82
• git diff --check