fix(docs): emit .md URLs in AI indexes + build-time cross-link validator

[tonytlwu]

Summary

• All AI discovery surfaces (llms.txt, llms-full.txt, agent-skills SKILL.md files, V3 library catalog docUrl) now emit raw .md URLs instead of .html. This makes Studio's fetch_fliplet_doc short-circuit work immediately — web_fetch refuses non-Google HTML, so every .html URL was guaranteed to waste a round-trip before falling back to .md anyway.
• Adds validateCrossLinks() to bin/build-agent-indexes.mjs — a fence-aware build-time link rot detector wired into --strict mode. Resolves ../, ./, /-rooted, bare, and .md/.html/extensionless links; skips external hosts and asset extensions. Escape hatches: <!-- lint-ignore-link --> on the offending link, or LINK_ALLOWLIST for known exceptions.
• Author docs updated in CONTRIBUTING.md ("Internal links" section) and CLAUDE.md (build pipeline note).

Context

Part of a two-lever fix for Studio V3 doc-fetch inefficiency (see companion Studio PR). Lever 2 here: once deployed to CF Pages, the index will serve .md URLs so the short-circuit fires even for new builder sessions that haven't loaded updated Studio code yet.

Root cause: apps-ai-web-fetch.js:388 returns USE_WEB_SEARCH for all non-Google HTML — a server-side design decision. Every .html doc URL from llms.txt was hitting this wall, costing a wasted fetch + fallback on every single doc lookup.

Verification: 157/157 tests pass (npm run test:unit). Regenerated .well-known/* artifacts have 0 .html entries.

Test plan

• cd docs && npm run test:unit — 157/157 pass
• node bin/build-agent-indexes.mjs — no errors, .well-known/llms.txt contains only .md URLs
• node bin/build-agent-indexes.mjs --strict — passes on clean tree
• CF Pages preview URL: verify /.well-known/llms.txt serves .md entries

🤖 Generated with Claude Code

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