/add-chapter

Add a new chapter to plan.md and task.md. Calculates next chapter number, creates section placeholders, and updates task checklist.

# Add Chapter to Learning Resource

Add a new chapter to the current learning resource project.

## Context

Current plan structure:
!`grep -E "^###? (Part|Chapter)" plan.md 2>/dev/null | head -30 || echo "[ERROR] plan.md not found - run /init-project first"`

Current task structure:
!`grep -E "^## Part|^### Chapter" task.md 2>/dev/null | head -20 || echo "[ERROR] task.md not found - run /init-project first"`

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## Parse Arguments

Extract from `$ARGUMENTS`:

- **Chapter Title**: The title for the new chapter (required)
- **Part Number**: Target part if specified with `--part` (default: ...