fix(site): preview lesson docs from the deploy's branch, not always main

[rohitg00]

Problem

lesson.html hardcoded the raw.githubusercontent main branch when fetching every lesson's markdown:

var rawUrl = 'https://raw.githubusercontent.com/.../main/' + path + '/docs/en.md';

So every deploy (production, PR preview, any branch) always rendered main's content. Content PRs (new lessons, the figure fences) could never be seen on their Vercel preview before merge.

Fix

• build.js writes site/build-meta.js with the build's git ref: VERCEL_GIT_COMMIT_REF on Vercel, local git rev-parse otherwise, falling back to main.
• lesson.html loads it and fetches docs + quiz from window.__AIFS_REF instead of a hardcoded main.
• build-meta.js is deploy-generated and gitignored (same pattern as sitemap.xml / llms.txt).

Production is unchanged: on main, the ref is main. PR previews now fetch from their own branch, so they render their own lesson edits.

Validation (headless, real lesson.html)

• GitHub raw resolves a branch ref with a slash: .../fix/branch-aware-doc-fetch/.../en.md → 200.
• window.__AIFS_REF = the branch; the lesson body and its qlearning-gridworld figure render from the branch's doc (mounted=1), zero blocking errors.
• The only 404 is the optional quiz.json for a quiz-less lesson, handled by the existing .catch (same as before).

After this merges, PR #282 (figures wave 3) and every future content PR will preview correctly on Vercel.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 8b348b03-8e6e-4c77-8610-c3a321c263ab

📥 Commits

Reviewing files that changed from the base of the PR and between 148de36 and e15ad57.

📒 Files selected for processing (3)

• .gitignore
• site/build.js
• site/lesson.html

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📝 Walkthrough

Walkthrough

The PR introduces a deployment metadata system that captures which git ref is being deployed and makes it available to the client. The build script generates a metadata file containing the git branch or commit ref, and the lesson page loads this metadata to fetch content from the correct GitHub branch instead of always using hardcoded main.

Changes

Deployment git ref metadata

Layer / File(s)	Summary
Build metadata generation and storage .gitignore, site/build.js	writeBuildMeta() reads VERCEL_GIT_COMMIT_REF environment variable or resolves the current git branch name (defaulting to main if empty or HEAD), then writes site/build-meta.js defining window.__AIFS_REF. The generated file is added to .gitignore.
Client-side metadata loading and usage site/lesson.html	The lesson HTML loads build-meta.js before other scripts, enabling fetchLesson to construct GitHub raw content and quiz URLs using the dynamic ref from window.__AIFS_REF instead of hardcoded main branch paths.

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🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: enabling lesson docs to be fetched from the deploy's branch instead of always using main.
Description check	✅ Passed	The description is directly related to the changeset, explaining the problem, solution, and validation for branch-aware lesson content fetching.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/branch-aware-doc-fetch

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