docs: Keep a Changelog rewrite + feature-parity PLAN.md

[mmcky]

Summary

Documentation-only PR with two deliverables from a deep review of the theme and a comparison against quantecon-book-theme, ahead of migrating the QuantEcon lectures to Jupyter Book ≥ 2.

1. CHANGELOG.md → Keep a Changelog format

Reconstructed from git history + PRs and reformatted to match quantecon-book-theme (Keep a Changelog + SemVer):

• [Unreleased] section capturing the post-1.1.1 work that is in main but not yet deployed to QuantEcon/quantecon-theme (still v1.1.1): @myst-theme 0.14→1.1.2 (Upgrade @myst-theme/* from 0.14.x to 1.1.2 #30), Remix 1.17→1.19 (Fix CI: bump @remix-run/* from 1.17 to 1.19 (peer dependency conflict) #29), Node >=18 + CI + a11y fixes (fix: quick wins from technical review (bugs, CI, Node, docs) #31, fix: resolve CI type-check errors (lodash types, Buffer→Uint8Array) #32), security overrides. Proposed release: 1.2.0.
• Backfilled 1.0.0→1.1.1 with grouped Added/Changed/Fixed entries, dates, and compare links.

2. PLAN.md → phased feature-parity plan

Maps quantecon-book-theme v0.20.3 features against this theme and lays out Phases 0–6.

Headline gap (flagship Phase 1): git history in lecture headers ("Last changed" + changelog dropdown).

Key architectural call-out: the MyST theme is a runtime Remix server with no access to the git repo, so git-derived data must be injected upstream via a MyST plugin at myst build time — the theme only renders it. Each affected phase therefore has an upstream deliverable plus the theme change.

Notes / decisions for review

• Changesets interaction: release.yml uses Changesets, which prepends to CHANGELOG.md in its own format. This hand-maintained Keep a Changelog file will need a decision (reconcile formats, or move the changelog to manual). Tracked as a Phase 0 item.
• This is docs-only — no code or dependency changes.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This documentation-only PR prepares for migrating QuantEcon lectures to Jupyter Book/MyST ≥ 2 by (1) standardizing the project changelog into a Keep a Changelog + SemVer structure and (2) adding a phased roadmap to reach feature parity with quantecon-book-theme.

Changes:

• Rewrites CHANGELOG.md into Keep a Changelog format with an [Unreleased] section and backfilled historical entries.
• Adds PLAN.md describing phases 0–6 to close feature gaps vs quantecon-book-theme, including build-time vs runtime constraints and upstream plugin needs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
PLAN.md	New phased feature-parity plan and architectural constraints for build-time git-derived metadata.
CHANGELOG.md	Keep a Changelog rewrite with unreleased + historical entries and reference links.

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[mmcky]

Addressed Copilot's 4 review comments in 96943d9 (docs-only). Threaded replies are on each comment so they can be resolved individually.

• PLAN.md:114 (zero-width char) — Fixed. Removed the stray U+200B; now a plain commit/history slash (confirmed it was the only zero-width char in either file).
• CHANGELOG.md:12 ("tagged from package.json") — Fixed. Reworded: the version comes from package.json and each release is a deploy commit (🚀 vX.Y.Z from <sha>) in the bundle repo; there are no git tags currently.
• CHANGELOG.md footer links (tag URLs 404) — Deferred, not changed. The links follow the Keep a Changelog standard, which assumes vX.Y.Z tags — the real gap is that the release workflow never creates them. Added a Phase 0 task in PLAN.md to introduce release tags (retroactively at the recorded source commits + going forward), after which the links resolve.
• CHANGELOG.md:43 (1.1.1 date) — Fixed, but with a different date than suggested. The actual release was the deploy to users on 2025-06-13 (56d1f0b in the bundle repo); every entry here is dated by deploy date. The 2026-02-25 date is when the source version-bump PR v1.1.1 #8 belatedly merged (bookkeeping). Corrected the header to 2025-06-13 and reworded the note, rather than using 2026-02-25.