feat(session): add fork/edit and revert actions with reliable revert diffs

[olibols]

Issue for this PR

Closes #9661
Closes #8885
Relates #10287
Relates #13481

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds desktop session controls for fork/edit/revert directly from message UI and hardens revert behavior for diff accuracy.

• introduces fork/edit/revert actions where users interact with messages
• improves revert internals to produce more reliable diffs
• updates labels/translations for the new actions

How did you verify your code works?

• Pre-push checks ran bun turbo typecheck successfully
• Verified fork/edit/revert actions and revert-diff behavior in desktop session flow

Screenshots / recordings

opencode-desktop_7eVfiV0XjL.mp4

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate PRs Found

I found several related PRs that might be addressing similar functionality:

1. PR feat(desktop): add Revert and Fork actions to user messages #9665 - feat(desktop): add Revert and Fork actions to user messages

   • This appears to be directly related as it adds Revert and Fork actions to user messages in the desktop app

2. PR feat(app,ui): add undo/fork buttons on user messages and agent description placeholder #13266 - feat(app,ui): add undo/fork buttons on user messages and agent description placeholder

   • Related to adding fork buttons on user messages UI

3. PR feat: add desktop timeline revert controls #13670 - feat: add desktop timeline revert controls

   • Related to revert controls in the desktop timeline

4. PR feat: message edit (undo) with confirm/cancel flow #12952 - feat: message edit (undo) with confirm/cancel flow

   • Related to message editing functionality with undo/revert capability

Note: PR #15291 (the current PR) appears in all searches, so these are the most relevant related PRs. You should review PR #9665 in particular, as it seems to be addressing the same feature set (fork and revert actions on user messages).