fix(ci): use anchored regex for lychee exclude-path patterns

[pdmack]

Summary

Use anchored regex patterns for lychee --exclude-path to prevent partial
directory name matches.

Motivation / Context

Bare path prefixes like docs/design would match docs/design-patterns or any
other path that starts with those characters. Anchored patterns
(^docs/design(/|$)) match only the exact directory and its contents.

Fixes: N/A
Related: N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Build/CI/tooling

Component(s) Affected

• Other: Fern docs CI (.github/workflows/fern-docs-ci.yaml)

Implementation Notes

Changed --exclude-path 'docs/design' to --exclude-path '^docs/design(/|$)'
(and same for docs/plans, docs/conformance) so exclusions are anchored to
the exact directory, not any path that starts with those characters.

Testing

CI-only change — no make qualify applicable.

Risk Assessment

• Low — Isolated change, well-tested, easy to revert

Rollout notes: N/A

Checklist

• Tests pass locally (make test with -race)
• Linter passes (make lint)
• I did not skip/disable tests to make CI green
• I added/updated tests for new functionality
• I updated docs if user-facing behavior changed
• Changes follow existing patterns in the codebase
• Commits are cryptographically signed (git commit -S)

[github-actions]

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