fix(ci): enable anchor link validation in scheduled link checker

[jsklan]

Description

Fixes a broken same-page anchor link in the overrides docs page, and updates the scheduled link checker to catch this class of bug going forward.

The link [multiple override files](#multiple-override-files) pointed to a heading that doesn't exist — the correct anchor is #managing-overrides-across-apis. The scheduled check-links.yml workflow didn't catch this because it explicitly excluded anchor-only links (^#) and didn't validate HTML fragment targets.

Context: The ^# exclusion and lack of --include-fragments were part of the original lychee setup in #2581, likely to avoid noise from fragments that couldn't be validated via HTTP. With --include-fragments, lychee now fetches each page and verifies that anchor targets exist as id attributes in the rendered HTML.

Changes Made

• overrides.mdx: Fix broken anchor #multiple-override-files → #managing-overrides-across-apis
• check-links.yml: Remove ^# exclusion so anchor links are no longer skipped
• check-links.yml: Add --include-fragments flag so lychee validates that anchor targets actually exist in rendered pages

Testing

• Trigger check-links.yml manually via workflow_dispatch to verify anchor validation works
• Confirm no false positives from the new fragment checking

[github-actions]

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• learn/api-definitions/openapi/overrides

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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