[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.

Start with `.claude/skills/speckit-implement/SKILL.md`, at the “Check checklists status” instructions, and compare its checklist parsing contract with `.claude/skills/speckit-clarify/SKILL.md`. Verify the behavior using a checklist containing checkbox examples inside a fenced code block and real checklist items outside it. Done means fenced examples are ignored while per-checklist total, checked, and unchecked counts remain correct.

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