ci.yml pins mystmd 1.9.1 with a comment that no longer describes the setup

[mmcky]

.github/workflows/ci.yml installs the MyST CLI as a pinned version with a comment explaining the choice, and both the pin's rationale and the names in it are now out of date:

# mystmd 1.9.x emits the notebook output-node AST expected by the
# quantecon-theme v2.0.0 bundle (@myst-theme 1.x); Node >=20 is required
# to run that theme during `myst build`. See QuantEcon/quantecon-theme-src.
npm install -g mystmd@1.9.1 thebe-core thebe thebe-lite

Three things to correct:

Claim	Reality
"the quantecon-theme v2.0.0 bundle"	the repo pins a much later theme; PR #78 takes it to v3.0.0
"See QuantEcon/quantecon-theme-src"	that repo was renamed to QuantEcon/quantecon-theme.mystmd on 2026-06-11 (old links redirect), and the separately-named QuantEcon/quantecon-theme bundle repo is archived
"Node >=20 is required"	still true as a floor, but the theme's own .nvmrc is Node 24, and the workflow sets node-version: 20.x

The pin itself is not broken. Theme v3.0.0 builds fine under mystmd 1.9.1 — the PR #78 preview rendered the default footer, the canonical link and the {raw}-rewritten page correctly — so this is documentation drift rather than a functional problem, and there is no urgency.

Worth deciding at the same time: whether the pin should move at all. The theme repo itself develops against the QuantEcon fork of the CLI (v1.10.1 (qe-v10)), and lecture-python-programming's jb2 branch builds from that fork's main rather than a published mystmd. This repo pinning a two-minor-versions-old upstream release is a deliberate-looking choice that the comment no longer explains.

Suggested fix: reword the comment to say what the pin is actually for, name the current theme repository, and either bump node-version to 24 to match the theme's .nvmrc or state why 20 is kept.

Noticed while taking theme v3.0.0 in #78.

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