Scope Prettier with .prettierignore and check formatting in CI (Phase A1)

[dknauss]

Summary

Phase A, ticket 1 of docs/planning/refactor-and-standards-memo.md.

Prettier had no ignore file, so npx prettier --check . scanned vendor/ and flagged 1,543 files. This PR:

• Adds .prettierignore. It excludes:
  • third-party, generated and build files;
  • lockfiles;
  • fixtures and snapshots, which are compared byte for byte;
  • Markdown, which is hand-wrapped prose using - list markers;
  • SCSS, which Stylelint owns. Prettier's SCSS output breaks 39 Stylelint rules.
• Reformats the 10 JSON/YAML files Prettier still flagged: issue templates, four workflows, codecov.yml, components.json and .planning/config.json. Each one parses to identical data before and after, checked with yaml.safe_load / json.loads.
• Adds npm run format:check (prettier --check .) and runs it in CI's Quality job, so formatting can't drift again. It's also noted in CLAUDE.md.

Validation

• npm run format:check: "All matched files use Prettier code style!"
• npm run lint:css: unchanged, since the SCSS files are untouched.
• python3 tools/check-components.py: passes on the reformatted components.json, which it only reads.
• Each reformatted file is data-identical to its previous version.

Checklist

• No behavior change
• CI enforces the new check

🤖 Generated with Claude Code

https://claude.ai/code/session_01KbVuMPN7YTcV27vwsG3LZ2

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.28%. Comparing base (4d638a2) to head (5e0ab7e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #150   +/-   ##
=========================================
  Coverage     88.28%   88.28%           
  Complexity       22       22           
=========================================
  Files            61       61           
  Lines          7290     7290           
  Branches        690      690           
=========================================
  Hits           6436     6436           
  Misses          291      291           
  Partials        563      563           

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