docs: add LLM test-writing skill and integrate test guidance

[icook]

Summary

• Add docs/skills/test-writing.md with decision tree for LLMs to choose correct test types and placement
• Update docs/playbooks/new-graphql-endpoint.md to use #[shared_runtime_test] with test_transaction() pattern
• Add consistent cross-links between all test guidance docs (backend-test-patterns, frontend-test-patterns, test-data-factories, fixing-flaky-tests, migration-testing)
• Link new test-writing skill from docs/README.md

Closes #223

Test plan

• Verify all relative links work correctly in rendered markdown
• Confirm test-writing.md appears in docs/README.md skills section
• Check cross-links navigate correctly between test docs

🤖 Generated with Claude Code

[github-actions]

📊 CI Reports

Report	Link
📊 Coverage	https://icook.github.io/tiny-congress/pr/248/coverage/
📚 Storybook	https://icook.github.io/tiny-congress/pr/248/storybook/
🎭 Playwright	https://icook.github.io/tiny-congress/pr/248/playwright/
📋 API Schemas	GraphQL · OpenAPI

_{🤖 Reports auto-generated from commit 734b149}