Add Windows compatibility path for ClawTeam

[Imichoo]

Summary

• add a native Windows compatibility layer for file locking and safe file replacement
• switch Windows defaults to a subprocess-based backend (windows) instead of tmux
• document local Windows usage and validated workflows
• add Windows helper scripts, security notes, caveats, and stronger production-style soak coverage

What changed

• replace Unix-only cntl assumptions with cross-platform helpers
• use Windows-safe replace semantics in config/team/session/cost/workspace/spawn-registry persistence paths
• add windows backend alias and make Windows default to subprocess mode
• make �oard attach fail cleanly on Windows and direct users to the web board
• add README-Windows.md, WINDOWS_RUNBOOK.md, and SECURITY-NOTES.md
• add PowerShell helpers for wrapper, spawn, board, session, smoke, and soak workflows
• add Windows-specific tests and update cross-platform test expectations

Validation performed on Windows

• team create/status
• task create/update/list with dependency unblocking
• inbox send/peek/receive
• cost budget/report/show
• session save/show
• team snapshot/list/dry-run restore
• workspace list/status/checkpoint/merge/cleanup
• spawn using the windows backend
• web board /api/overview check returns HTTP 200
• production-style soak with repeated worker spawn + workspace merge + session/inbox cycles

Caveat

• tmux-native tiled pane orchestration remains Unix/Linux-first upstream behavior
• on Windows, the practical equivalent is the subprocess/windows backend plus the web board