Consolidate dependency files (environment.yml vs requirements.txt vs requirements-build.txt)

Summary

There are three overlapping dependency manifests that disagree on the details, and only one of them is actually used. This makes it easy for deps to rot and — more importantly — means a local setup that follows the README can't run the JAX lectures.

The three files

grep confirms nothing in .github/, scripts/, or README.md references either requirements*.txt. CI builds the env from environment.yml, then installs JAX out of band via pip install -U jax (ci.yml, publish.yml).

Problems

1. No single source of truth — three files, two of them orphaned and liable to rot.
2. Redundancy via the anaconda metapackage — anaconda already bundles numpy, matplotlib, and numba, so the explicit lines in the requirements files duplicate it.
3. numba is listed everywhere but unused (via anaconda + both requirements files) — see #2 (Numba advertised but unused).
4. Likely channel typo — environment.yml declares channels: - default (singular); the canonical conda channel is defaults, which the other QuantEcon lecture repos use.

Suggested direction

Pick one source of truth. Two QuantEcon-friendly shapes:

• Conda-first (matches CI): add jax to environment.yml, drop the unused numba, fix defaults, and delete both requirements*.txt. The separate CI pip install -U jax could then go away.
• Keep a minimal requirements.txt for the Colab / "just run the notebooks" path (matplotlib, numpy, jax), referenced from notebooks/README.md, and make environment.yml the single build source of truth (remove requirements-build.txt).

Either way: one runtime manifest + at most one build manifest, JAX listed where the README points, and the channel name corrected.

Open question before any consolidation

Related: #2 (numba unused), #5 (GPU runner / RunsOn).

Start by comparing environment.yml, requirements.txt, and requirements-build.txt with the CI references in .github/ci.yml and publish.yml, then check README.md and notebooks/README.md. Review related issues #2 and #5 before choosing a consolidation shape. Done means the selected manifests are the only maintained dependency sources, JAX is listed where the setup instructions point, and CI no longer relies on redundant installation steps.