feat: Enable GitHub header actions in MkDocs documentation (#1933)

Add content.action.edit and content.action.view features to mkdocs.yml
to enable GitHub integration links in the documentation header bar.

These Material for MkDocs features automatically create:
- "Edit this page" button linking to GitHub source
- "View source" button for file viewing
- GitHub repository icon in header (via existing repo_url config)

Changes:
- mkdocs.yml: Added content.action.edit and content.action.view to theme.features
- docs/development/case-studies/issue-1931-proportionality.md: Root cause analysis
  documenting initial over-engineering (58 tests for 2-line change) and prevention
  mechanisms for future workflow improvements

Testing:
- mkdocs build --clean --strict: SUCCESS
- Pre-commit hooks: All passed
- Config validation: Confirmed valid YAML
- Manual verification: GitHub links will appear via Material theme built-in feature

This was initially over-engineered with elaborate testing (58 tests, 29K lines).
After user feedback, we applied ruthless simplicity: just the 2-line config change
needed, with verification via mkdocs build. The analysis document captures lessons
learned about proportionality and workflow improvements needed.

Closes #1931

Co-authored-by: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>

Author: rysweet