docs: clarify link checking rules in AGENTS.md

[devin-ai-integration]

Summary

Tightens the "Link checking" section of AGENTS.md so agents constructing internal URLs don't get tripped up by cases the current doc glosses over:

• Adds skip-slug: true to the URL segment table and callout. It's used in fern/products/docs/docs.yml (e.g., the nested Navigation section) and fern/products/api-def/api-def.yml, so walking the section hierarchy naively will produce broken URLs.
• Generalizes the Home product footnote: any product/section/tab with no slug: (or with skip-slug: true) is omitted from the URL, not just Home.
• Clarifies slug precedence in the "Steps to construct a link" list: page frontmatter slug: wins over slug: in the navigation YML, which wins over the auto-derived value.

No content changes outside AGENTS.md.

Review & Testing Checklist for Human

• Read through the updated "Link checking" section in AGENTS.md and confirm the new wording matches how you want agents to reason about URLs (especially the frontmatter > YAML slug precedence and the skip-slug: true handling).

Notes

Scope is intentionally small — this only edits AGENTS.md. No docs pages or config changed, so there's nothing meaningful to preview on the site.

Link to Devin session: https://app.devin.ai/sessions/a135a71acfa04edab4df6daa04f90a27
Requested by: @devalog

[devin-ai-integration]

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