Fix GFM table rendering in GitHub PR descriptions

[ymichael]

Summary

• render GitHub Flavored Markdown tables as structured, overflow-safe tables in GitHub issue and pull request bodies
• support alignment markers, optional outer pipes, escaped pipes, and existing inline formatting inside cells
• add focused frontend regression coverage based on the table from Pin GitHub Actions to immutable commit SHAs #1141

Testing

• pnpm exec turbo run test typecheck build --filter=bb-plugin-github --force
• pnpm exec prettier --check plugins/github/components/markdown-lite.tsx plugins/github/components/markdown-lite.test.tsx plugins/github/vitest.config.ts plugins/github/package.json
• pnpm install --lockfile-only --frozen-lockfile
• git diff --check

Fixes #1142