Add native Windows tmux backend

[Tim-Machine]

Summary

• Add WindowsTmuxMultiplexer for native Windows hosts using tmux-windows.
• Rewrite tmux-windows absolute -c cwd arguments to -c . while spawning tmux from the requested cwd.
• Inject inherited Windows environment into agent windows so CLI binaries like codex resolve inside panes.
• Use PowerShell syntax for parked control windows and document the pipe-pane no-op tradeoff on Windows.

Verification

• PYTHONUTF8=1 uv run --all-extras --group dev pytest tests/test_tui_launch.py tests/test_multiplexer.py tests/test_backend_registry.py tests/test_portability_guard.py -q -> 65 passed
• uv run --group dev ruff check src/bmad_loop/adapters/tmux_windows_backend.py src/bmad_loop/adapters/multiplexer.py tests/test_multiplexer.py tests/test_backend_registry.py tests/test_portability_guard.py tests/test_tui_launch.py -> pass
• git diff --check -> pass
• Local source install: bmad-loop validate --project C:\DevProjects\zerg -> WindowsTmuxMultiplexer available, codex found, hooks registered
• Full suite collection: PYTHONUTF8=1 uv run --all-extras --group dev pytest --collect-only -q -> 1512 tests collected

Notes

• A full pytest -q run on the Windows workstation exceeded a 184s command timeout before producing buffered output, so this PR is draft pending CI/full-suite confirmation.
• pipe_pane is deliberately a no-op for this backend because live tmux-windows tests showed pipe-pane can terminate the tmux server when spawning Windows pipe commands.

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[pbean]

Thanks for your submission! It's still currently in draft so unsure if that was intentional, if so when you are finished with your work. Please go ahead and remove all the documentation changes you made just keep the adapter code and tests. We will work on adjusting the documentation if the PR is merged. Also, at this time we have updated how multiplexers are selected and managed per issue #87's request so ensure compatibility with the newest changes in main.