fix(vscode): keep prompt undo and redo inside the webview

[marius-kilocode]

What Problem This Solves

Undo and redo in the focused chat input can change the underlying Markdown file when a Markdown custom editor is active instead of changing the draft.

Closes #13724.

Why This Change Was Made

VS Code forwards webview undo/redo shortcuts to the host, where custom-editor command handling can take priority over the focused webview. Stop those keyboard events at the prompt textarea without preventing native text undo/redo. Keep the existing enhanced-prompt undo and speech shortcut handling.

User Impact

Undo/redo stays in the focused prompt with Markdown Preview or Markdown Editor open. This adds no features, settings, or global keybindings. Clipboard shortcuts, Alt-modified combinations, and Shift+Y retain their existing routing. Custom VS Code commands assigned to the intercepted undo/redo combinations no longer receive those events while the prompt has focus.

Evidence

• Extension build, host/webview type checks, lint, formatting, Knip, and the Kilo change-marker guard passed.
• 75 nearby prompt/history/mention tests passed. Type checks, lint, and these tests also passed after rebasing onto main.
• Three Chromium regression tests passed against a fresh Storybook build: native undo/redo despite host forwarding, shortcut boundaries, and enhanced-prompt undo.
• Real isolated macOS VS Code checks passed with plain text, Markdown Preview, and Markdown Editor active, including unlocked Markdown editing. Prompt undo/redo worked while the document remained unchanged. The isolated instance was cleaned up.
• Windows-specific verification remains outstanding. Check Ctrl+Z and Ctrl+Y with each Markdown custom editor active; only the focused prompt should change.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• .changeset/chat-input-undo.md
• packages/kilo-vscode/tests/prompt-undo.spec.ts
• packages/kilo-vscode/webview-ui/src/components/chat/PromptInput.tsx

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_{Review guidance: REVIEW.md from base branch main}