/add-section

Add a new section to an existing chapter. Calculates section number (e.g., 5.4), adds subtopic placeholders to plan.md, and updates task.md checklist.

# Add Section to Chapter

Add a new section to an existing chapter in the learning resource project.

## Context

Target chapter structure (use $1 as chapter number from parsed arguments):
!`grep -A 15 "^### Chapter" plan.md 2>/dev/null | head -40 || echo "[ERROR] plan.md not found - run /init-project first"`

Current sections in project:
!`grep -E "^#### [0-9]+\.[0-9]+" plan.md 2>/dev/null | tail -10 || echo "[No sections found]"`

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## Parse Arguments

Extract from `$ARGUMENTS`:

- **Section Title**: The title for the new section (required)
- **Chapter Number**: Target chapter with `--...