docs(useeffect): add 'adjust state during render' pattern

[whinc]

Resolves #24

Summary

• Add missing case from React official docs: "Adjusting some state when a prop changes"
• Add new section "Adjust State During Render (Partial Reset)" with prevItems pattern
• Update Quick Reference table and Decision Tree to distinguish between reset ALL state vs adjust SOME state

What's Changed

SKILL.md:

• Added Quick Reference entry for partial state reset scenario
• Expanded Decision Tree with two branches: "Reset ALL state?" and "Adjust SOME state?"

alternatives.md:

• Added new Section 4: "Adjust State During Render (Partial Reset)"
• Includes BAD/BETTER/BEST code examples
• Documents important constraints and priority order
• Re-numbered subsequent sections (5-9)
• Updated Summary table

Why This Matters

The skill previously only covered:

• key prop (resets ALL state)
• Store ID and derive object (calculate during render)

This PR adds the middle ground: adjusting state during render when you need to reset some (not all) state when a prop changes, filling the gap between the two existing approaches.

Test plan

• Verified skill content matches React official documentation
• Code examples are syntactically correct
• Section numbering is consistent