feat(dev): add explicit tmux multiplexer launch

[basnijholt]

Summary

• add -m tmux / --multiplexer tmux for dev new and dev agent
• allow tmux-backed agent launches even when not already inside tmux by creating or reusing a detached repo-scoped session
• return and surface the tmux pane handle in CLI output, and update docs/tests accordingly

Testing

• ruff check agent_cli/dev/launch.py agent_cli/dev/terminals/tmux.py tests/dev/test_launch.py tests/dev/test_terminals.py
• pytest tests/dev/test_terminals.py tests/dev/test_launch.py tests/dev/test_cli.py -q