Phase 2: deploy on mystmd — strict static build, pinned theme and plugin URLs

[mmcky]

Part of #28 (Phase 2). Gated on the report theme's v1.0.0 (QuantEcon/quantecon-theme-report.mystmd#2).

Problem. .github/workflows/deploy.yml at a92f911b runs jupyter-book build lectures and uploads lectures/_build/html to GitHub Pages. The mystmd equivalent is a static myst build --html, which is also the path where the lecture theme's stylesheet-asset defects hid (quantecon-theme.mystmd#138 and #150), so it must be exercised on a preview before it becomes the deploy.

Proposed work.

• deploy.yml: install mystmd at a pinned version, run myst build --html --strict from lectures/, upload lectures/_build/html; keep the Pages environment and concurrency settings; drop the Python setup once Phase 1: native charts page — directives replace the matplotlib cells #33 removes execution.
• myst.yml pins: site.template to the report theme's v1.0.0 zip, project.plugins to the plugins repo's datavis.mjs release asset, the report theme's compliance.mjs release asset, and the lecture theme's git-metadata.mjs URL; fetch-depth: 0 so per-page history is complete.
• A dry run on the mystmd branch through the report theme's preview workflow, then a Pages deploy from a manual workflow_dispatch before the branch merges.
• Retire _config.yml, _toc.yml and the Jupyter Book requirements after the first successful deploy.

Acceptance criteria.

• The published site is built by myst build --html --strict and renders the five page types with the report theme
• Deleting a CSV on a branch fails the deploy workflow
• All stylesheet and font assets resolve on the live site (no 404s in a link-checker pass)
• The Jupyter Book configuration is removed and README.md points at the new build