Expand internal link linting coverage

[KayleeWilliams]

Context

Leadtype already catches stale root-relative /docs/... links with the invalid-link rule. If a page such as docs/acme.mdx or docs/acme/index.mdx is removed, surviving references to /docs/acme should fail lint.

Proposal

Add or verify coverage for internal link cases that may bypass the current route check:

• relative internal links such as ../acme or ./acme
• hash anchors such as /docs/acme#install, including missing heading anchors
• custom docs mount prefixes beyond /docs
• curated/config-owned links that point at docs pages, if they bypass page markdown/frontmatter checks

Acceptance criteria

• Stale page links continue to report invalid-link.
• Each supported internal link shape has regression tests.
• Unsupported or non-goal link shapes are documented clearly in the lint reference.

[pullfrog]

  ^{｜ Build this ➔ ｜ Make a plan ➔}

[coderabbitai]

🔗 Related PRs

#13 - Overhaul docs IA, ship AGENTS.md for bundled docs, add eval harness [merged]
#27 - Add Table of Contents extraction, sidebar, and authoring docs [merged]

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