setup-environment: fix standard-mode conda caching + validation (#33)

[mmcky]

Closes #33 (C3, L19, L23) — makes the non-container (standard-mode) Conda cache actually save time, plus two smaller hardening/doc fixes.

C3 — the cache was a no-op

Before, standard mode restored the Conda cache and then ran setup-miniconda with environment-file: set, which recreates the env every run — so the cache never helped (the README's "~3-4 min cached" was aspirational).

Restructured to the documented setup-miniconda caching recipe:

1. setup-miniconda runs without environment-file (installs conda + the named env shell, doesn't rebuild from the file);
2. actions/cache restores the env at ${CONDA}/envs;
3. a new step runs conda env update only on a cache miss (cache-hit != 'true', which also covers prefix-matched restores when the env file changed).

Also:

• dropped the deprecated use-only-tar-bz2;
• dropped miniconda-version: latest so the env lives at a stable ${CONDA}/envs path (the runner's pre-installed conda, kept current by auto-update-conda) — this is what makes the cache path reliable.

L19 — fail loudly on missing LaTeX requirements

install-latex: true with a missing requirements file now errors and exits instead of warning + skipping (which produced an env that failed ~an hour later in the build).

L23 — cache-version doc

Documented that cache-version only affects the standard-mode Conda cache (no effect in container mode, which has no Conda cache).

⚠️ Validation note

Standard mode is a fallback (the templates are container-first), and conda env caching is finicky and can't be verified locally. Worth confirming on a real CI run: the second run on an unchanged environment.yml should log a cache hit and skip the "Create/update environment" step. If the ${CONDA}/envs path turns out wrong on the hosted runner, the fix is a one-line path tweak.

Closes #33

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the setup-environment composite action to make standard (non-container) Conda caching effective, hardens LaTeX requirements handling, and documents the cache-version input’s scope.

Changes:

• Reworks standard-mode Conda setup to restore ${CONDA}/envs from cache and only update/create the env when the primary cache key doesn’t hit.
• Fails fast when install-latex: true but the LaTeX requirements file is missing.
• Documents that cache-version only affects the standard-mode Conda cache (no effect in container mode) and records the behavior in the changelog.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
setup-environment/action.yml	Reorders standard-mode Conda setup and caching; adds cache-miss update step; makes missing LaTeX requirements an error; clarifies cache-version input description.
CHANGELOG.md	Adds an Unreleased entry describing the setup-environment caching/validation/doc changes.