fix: restore TPM checkout after local sync

[corwinm]

Summary

• add npm run restore-tmux to return the TPM-installed checkout to a clean Git state after local development syncs
• preserve the installed node_modules while removing files copied by sync-tmux
• document when to restore before running TPM updates
• add a regression test covering sync → restore and dependency preservation

Root cause

npm run sync-tmux intentionally copies the development checkout over TPM's Git checkout at ~/.tmux/plugins/coding-agents-tmux. That leaves tracked files modified, so TPM's rebasing update refuses to pull. The development work was already committed upstream; only the copied files remained dirty in the installed checkout.

Validation

• npm run shell:check
• npm run fmt:check
• npm run lint
• npm run typecheck
• npm test (128 passing)
• restored the real TPM checkout and fast-forwarded it from 040361b to 80c8aaf