fix: make undo plan-aware

[wbxl2000]

Related Issue

No linked issue. This PR archives the plan-aware undo fix discussed in review.

Problem

/undo only rewound conversation context. In plan mode, it could remove the turn that entered plan mode while leaving plan mode active, so the UI and the next prompt stayed in plan mode with a half-written plan still present.

What changed

• Made undo cancel plan mode when it removes the turn that entered plan mode.
• Preserved manual /plan on behavior when undoing later prompts.
• Added tests for plan-aware undo and replay.
• Added a changeset.

Tests were added but could not be run in this environment because node and pnpm are unavailable.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: 4ccfc26

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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