docs: add CLAUDE.md and refresh stale parts of copilot-instructions.md

Background

We have `.github/copilot-instructions.md`
documenting project conventions for GitHub Copilot. We don't currently have
an equivalent for Claude Code, which auto-loads `CLAUDE.md` at the repo
root. Adding one makes Claude sessions in this repo aware of the same
project rules (tox-only-for-tests, asset-compilation workflow, release
process) without having to rediscover them every session.

While we're in there, the existing copilot-instructions.md has drifted from
the current state of the repo and needs a refresh.

Scope

1. Add a standalone `CLAUDE.md` at the repo root

Full content, written for Claude Code's idioms (no `tee /tmp` pattern,
HEREDOCs over create_file for gh commands, file_path:line_number
references, TodoWrite mentions for multi-step work). Mirrors the structure
of copilot-instructions.md so the two stay parallel on shared facts.

Should cover:

• Project overview
• Bootstrap (Python 3.13+, Node, tox, pre-commit, npm install)
• Build / test commands and timing
• Asset-compilation workflow (`npm run build` after editing
  `src/quantecon_book_theme/assets/`)
• Visual regression tests (new — uses
  `quantecon-book-theme-fixtures`)
• Code quality (pre-commit, flake8, black)
• Release process
• Project structure

2. Refresh `.github/copilot-instructions.md`

• Add a visual-regression-tests section covering the fixtures repo,
  `tox -e visual`, and the `/update-new-snapshots` / `/update-snapshots`
  PR-comment workflow. This was missing entirely.
• Update the `package.json` snippet at line ~182. Currently shows
  webpack 5.0.0 / sass 1.59 / css-loader 6.8 — actual versions are webpack
  5.103 / sass 1.99 / css-loader 7.1, plus dev-only additions
  (playwright/test, html-webpack-plugin, dedent, etc.).
• Update the directory listing at line ~160 to include
  `playwright.config.ts`, `.nvmrc`, and remove the inaccuracy that
  `tests/test_build/` is the only thing under `tests/` (there's now
  `tests/visual/`).
• Optional: consider trimming the Copilot-specific shell-escaping
  workarounds if they're stale.

Acceptance

• `CLAUDE.md` at repo root, full standalone content
• Visual-test / fixtures section added to copilot-instructions.md
• Stale package.json + directory listing fixed in copilot-instructions.md
• Both files agree on shared facts (build commands, test rules, release process)

Context

• Visual regression migration: #390
• Fixtures repo: https://github.com/QuantEcon/quantecon-book-theme-fixtures

🤖 Generated with Claude Code

Start by reading .github/copilot-instructions.md, package.json, and the existing tests/visual/ workflow, including tox -e visual and the fixtures repository. Add the root CLAUDE.md and update the Copilot guide so both document the current bootstrap, build, visual-test, quality, release, and project-structure facts. Done means the acceptance checklist passes and the two guides agree on shared facts.