feat: preview undo ranges before confirmation

[RealKai42]

Related Issue

No linked issue. This change follows a maintainer request to make /undo confirmable in the TUI instead of executing immediately.

Problem

/undo currently withdraws history as soon as the command is submitted. That makes it too easy to remove more conversation context than intended, and there is no visual confirmation step for choosing how many recent prompts should be withdrawn.

What changed

• Changed /undo to open a TUI-only preview mode instead of calling undoHistory immediately.
• Added keyboard handling so ↑ expands the undo range, ↓ narrows it, Enter confirms, and Esc / Ctrl-C / Ctrl-D cancels without touching core state.
• Rendered the selected undo turn with a compact dark-red preview that respects compaction boundaries and restores the withdrawn prompt into the input box after confirmation.
• Documented /undo [<count>] in the English and Chinese slash command reference.
• Added a changeset for the CLI package.

Verification

• pnpm --filter @moonshot-ai/kimi-code exec tsc --noEmit
• git diff --check
• pnpm --dir docs build was attempted but could not run because the docs package dependencies are not installed in this workspace (vitepress: command not found).
• gen-docs was used for the docs update, but its changelog sync prerequisite script docs/scripts/sync-changelog.mjs is not present in this checkout.

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: d125bb6

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	Minor

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