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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