fix(windows): fallback attach for psmux sessions

[camden-lowrance]

Summary

Fallback to plain tmux attach on Windows when attach-session -t fails under psmux.

Repro

• PowerShell on Windows with tmux.exe available via psmux
• gt mayor start succeeds
• tmux -L list-sessions shows hq-mayor
• gt mayor attach or tmux -L attach -t hq-mayor fails with:
  • psmux: can't find session 'hq-mayor' (no server running)

Change

• Keep the normal Windows attach path first
• If attach-session -t fails and we are not already inside the same tmux socket, retry with plain attach
• This matches observed psmux behavior where session listing works but targeted attach does not

Verification

• Built a Windows binary from current source
• Verified from PowerShell that:
  • gt mayor start succeeds
  • gt mayor attach no longer fails with the psmux session-not-found error

Notes

This PR contains only the Windows attach helper change.