[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.

从 `.claude/skills/speckit-implement/SKILL.md` 中的“Check checklists status”指令开始，并将其 checklist 解析契约与 `.claude/skills/speckit-clarify/SKILL.md` 进行比较。使用一个包含 fenced code block 内复选框示例以及其外部实际 checklist 项目的 checklist 来验证行为。完成标准是忽略 fenced code blocks 中的示例，同时每个 checklist 的总数、已勾选数和未勾选数仍保持正确。

由索引模型根据 Issue 内容生成。