Batch 4: Lint docs/design/ — resolve MD036 story-list pattern

[lisa-tarbo]

Part of #27.

3 files, 10 issues, all in docs/design/unified-assessment.md.

Steps

• Investigate: unified-assessment.md uses bold **Story N — ...** lines instead of headings for ~10 user-story summaries. Decide whether that's intentional (add a scoped MD036 exception) or should become real subheadings (better for an agent scanning/linking the doc — probably the right call)
• Apply the decided fix
• Remove docs/design from .markdownlintignore
• Verify: npx markdownlint-cli2 --config .markdownlint.yaml "docs/design/**/*.md" is clean