CI: check the manifest and content files agree

Add a check that content/course-manifest.json and the markdown files agree.

Why

The manifest declares 87 modules; 17 files exist. That gap is intentional (#16), but nothing distinguishes "intentionally unwritten" from "the file is named wrong and the page 404s". lib/content.ts resolves a module to content/<track>/<level>/<index>.md and returns null when the file is missing, so a mismatch is a silently broken page.

This will matter more with every module written, not less.

Scope

A small node script, run in CI, asserting:

• every content file has a corresponding manifest entry (a file nothing points at is dead weight)
• every manifest entry either has a content file or is explicitly marked as unwritten

The second half implies a convention for "planned but unwritten" — a flag in the manifest, or an allowlist. Deciding that is part of this issue.

Done when

A renamed or misplaced content file fails CI instead of producing a blank page.

Part of #38.

Start by reading content/course-manifest.json and lib/content.ts to confirm how manifest entries map to content///.md, then inspect how CI runs Node scripts. Define the convention for planned-but-unwritten modules and make the CI check fail for orphaned files or manifest entries that are neither present nor explicitly unwritten.