feat: reversible thread inbox state operations

[gnapse]

Summary

This PR adds reversible thread inbox-state operations and makes thread done safely undoable.

New Commands

• tw thread reopen [thread-refs...]
• tw thread read [thread-refs...]
• tw thread unread [thread-refs...]
• tw thread restore [thread-refs...] --unread

tw thread done now uses the same mutation flow as the commands above.

Mutation Behavior

• Supports single and bulk refs (tw thread read 123 456).
• Supports --yes, --dry-run, and --json.
• Idempotent operations (reopen on open threads, unread on unread threads).
• --json includes before/after fields: id, title, isArchived, inInbox, isUnread, closed, lastUpdated.
• Non-zero exit only for true errors (invalid ref, permission, API failure), including mixed success/failure batches.

Docs and Skill Content

• Updates README.md with examples and migration note: thread done is reversible via thread reopen.
• Updates src/lib/skills/content.ts to reflect new thread commands and options.

Tests

• Expands thread command tests to cover:
  • single and bulk operations
  • dry-run behavior
  • idempotency
  • mixed success/failure

[doist-bot]

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• Delivery: 2ab36b20-1750-11f1-8790-ba820cc8e210
• Logs: Datadog

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