docs: add a reference page for every lint finding code #5385

Problem

The linter has 120 finding codes, but there is no reference page for them. 70 of the codes are not mentioned anywhere in docs/ or skills/ (for example self_closing_media_tag, gsap_timeline_not_registered, caption_transcript_not_inline, media_missing_src). When hyperframes lint or check prints one, the user has only the message itself to go on.

docs/packages/lint.mdx has a short "What the Linter Catches" section, but it describes categories, not individual codes.

Proposed solution

Add a lint rule reference page (for example docs/reference/lint-rules.mdx) and link it from docs/packages/lint.mdx and the navigation in docs/docs.json.

For each code: severity, one sentence on what it catches, and how to fix it. Group them by the rule file they come from (packages/lint/src/rules/*.ts), since that already groups them by topic (captions, media, gsap, composition, fonts, textures, slideshow, structure, core, adapters, linked clips).

To list every code: git grep -hoE 'code: "[a-z_]+"' -- packages/lint/src/rules | sort -u

Acceptance criteria

• Every code in packages/lint/src/rules/*.ts (excluding tests) appears on the page with its severity and a fix.
• The page is reachable from the docs navigation and from docs/packages/lint.mdx.
• The docs checks pass: npx mint validate and npx mint broken-links from docs/ (what .github/workflows/docs.yml runs).

It's fine to split this into several PRs by rule file. Please say which files you're taking in a comment so two people don't write the same section.