[Bug]: /speckit-implement counts checkbox markers inside fenced code blocks — example checkboxes can falsely block implementation

## Version

spec-kit v1.0.1 (`.claude/skills/speckit-implement/SKILL.md`, "Check checklists status")

## Description

The checklist scan counts every `- [ ]` / `- [x]` / `- [X]` line in every checklist file, including lines inside fenced code blocks. A checklist that *documents* the checkbox format with an example fence therefore reports phantom unchecked items, and implementation is blocked by checkboxes that aren't real work items.

The clarify command already handles this correctly — `.claude/skills/speckit-clarify/SKILL.md` specifies parsing Markdown outside code fences — so this is an internal inconsistency between the two commands' parsers as much as a bug.

## Expected behavior

Count checkbox markers only outside fenced code blocks (same contract as the clarify parser), keeping the existing per-checklist total/checked/unchecked reporting.

`.claude/skills/speckit-implement/SKILL.md` の「Check checklists status」の指示から始め、チェックリストの解析契約を `.claude/skills/speckit-clarify/SKILL.md` と比較してください。fenced code block 内にチェックボックスの例があり、その外側に実際のチェックリスト項目があるチェックリストを使って、動作を確認してください。fenced code blocks 内の例が無視され、各チェックリストの合計、チェック済み、未チェックの件数が正しいままであれば完了です。

索引モデルが issue の本文から書いたものです。