Walk Track: Add doc link-checker CI step

[nikhil2611]

Context

Walk Ex4 and Ex11 significantly expanded the documentation (docs/architecture.md, docs/onboarding-walk.md, CONTRIBUTING.md, ai-track-docs/). No CI step validates internal cross-references or external URLs — broken links accumulate silently.

Code Paths

• .github/workflows/link-check.yml — new advisory workflow using lychee-action
• docs/, ai-track-docs/, CONTRIBUTING.md, README.md

Acceptance Criteria

• .github/workflows/link-check.yml runs lychee on all .md files on every PR
• Job is continue-on-error: true (advisory, non-blocking)
• .lycheeignore file excludes known-good but rate-limited URLs (e.g., GitHub anchors, localhost)
• At least one broken link found and fixed (or justified ignore)

Dependencies

None.

Size

Small.