feat(cli): add a non-interrupting /btw command

[c8dhjp4tyv-bit]

Recreated on the rewritten main after #1103 was auto-closed during repository maintenance. This carries the final reviewed branch state onto the new history while preserving upstream changes added after the rewrite.

Summary

• add /btw <additional thought> to send context without interrupting an active turn
• reuse the existing queue while preserving pending attachments
• send directly when idle and show usage for an empty note
• add slash-command metadata and regression tests
• retain the focused idle-attachment handoff regression added after review on the original PR

Fixes #1052

Validation

Prior validation before the history rewrite:

• targeted prompt, command, and router tests: 81 passed
• full CLI suite: 2,368 passed, 9 skipped
• full suite still reports 34 existing release-wrapper failures and 32 environment/harness errors
• CLI typecheck reaches existing missing tar and react-dom/server declarations; no errors point to the changed files
• git diff --check passed

The port onto rewritten main was three-way applied. Five files applied cleanly; the only drifted file was command-registry.ts, where the /btw hunk was reapplied while preserving the new upstream buildSkillPrompt import.