Add keyboard shortcut to reset Codex thread (clear history)

[HenryQuillin]

Problem

When Sage's Codex thread accumulates too much context, users see this error:

Codex ran out of room in the model's context window. Start a new conversation or clear earlier history before retrying.

Currently, the only way to clear history is to manually delete files:

rm -rf ~/.sage/YOUR-PROJECT-PATH/threads/
rm -rf ~/.sage/YOUR-PROJECT-PATH/reviews/

This is not discoverable and breaks the user's flow.

Proposed Solution

Add a keyboard shortcut (e.g., N for "new thread") that:

1. Deletes the current session's thread metadata (~/.sage/{project}/threads/{sessionId}.json)
2. Optionally clears the review cache (~/.sage/{project}/reviews/{sessionId}.json)
3. Creates a fresh Codex thread
4. Shows a confirmation message like ✓ Thread reset - starting fresh

Suggested UX

• Shortcut: N (available from the running screen)
• Confirmation: Optional prompt "Reset thread and start fresh? (y/n)" to prevent accidental resets
• Feedback: Brief status message confirming the reset

Implementation Notes

Key files:

• src/ui/App.tsx - Add keyboard handler in useInput
• src/lib/threads.ts - Add deleteThreadMetadata() function (or use existing delete logic)
• src/lib/review-cache.ts - Optionally clear cached reviews

The thread deletion logic already exists for edge cases (see thread-persistence.md documentation).

Additional Context

This came up from a user in Discord who hit the context limit and didn't know how to recover.