fix(vscode): move "Browse files..." to end of @-mention dropdown

[AmariahAK]

Fixes #12182

Issue

When using @-mention in the VS Code prompt, the "Browse files..." option was always the default selection instead of the closest matching file. Pressing Enter would open the native file picker rather than inserting the matched file.

Context

This regression was introduced in v7.4.7 alongside the "Browse files..." feature. The FILE_PICKER_RESULT entry was placed before the actual file matches in the results array, so it occupied index 0 and became the default selection.

Implementation

One-line swap in buildMentionResults — moved FILE_PICKER_RESULT from before ...results to after. Actual file matches now sit at index 0, making them the default selection. "Browse files..." remains always available at the bottom of the dropdown list.
This approach was chosen over alternatives:

• Changing mentionIndex to start at 1 — fragile when terminal/git-changes entries are also present
• Filtering out the file picker — reduces discoverability of a useful feature
  Files changed:
• packages/kilo-vscode/webview-ui/src/hooks/file-mention-utils.ts — swapped order
• packages/kilo-vscode/tests/unit/file-mention-utils.test.ts — updated 7 assertions to match
• .changeset/file-picker-default-selection.md — patch changeset for kilo-code

How to Test

Manual/local verification

• Launch the extension: bun run extension from repo root
• In the prompt input, type @ followed by a partial filename
• Press Enter — the closest matching file is selected, not the file picker
• Arrow down still reaches "Browse files..." at the bottom of the list

Reviewer test steps

1. cd packages/kilo-vscode
2. bun test ./tests/unit/file-mention-utils.test.ts — 67 pass, 0 fail
3. bun run typecheck — clean
4. Optional: bun run extension and test @-mention manually as described above

Checklist

• Issue linked above
• Tests/verification described
• Screenshots/video N/A — purely behavioral fix (no visual UI change)
• Changeset added
• I personally reviewed the diff and can explain the changes

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Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• .changeset/file-picker-default-selection.md
• packages/kilo-vscode/tests/unit/file-mention-utils.test.ts
• packages/kilo-vscode/webview-ui/src/hooks/file-mention-utils.ts

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_{Reviewed by gpt-5.6-sol · Input: 53.8K · Output: 4.9K · Cached: 253.2K}

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