Validate tmux popup workflow

[tnez]

Context
The primary expected invocation is a tmux popup or floating terminal surface. See docs/v1-scope.md and the PRD.

Acceptance criteria

• Document a working tmux popup command or keybinding.
• Verify hud opens comfortably in a tmux popup.
• Verify quit behavior returns control cleanly to tmux.
• Note any terminal sizing assumptions or minimum useful dimensions.
• README includes tmux popup usage.

Out of scope

• Replacing tmux.
• Managing tmux sessions directly unless configured as a normal action.

[tnez]

Implementation is complete in PR #11: #11. I could not close this issue from here because merging PR #11 into main was blocked pending explicit approval for the default-branch mutation. The PR is green and links this issue for automatic closure on merge.