feat(links): auto-remap base-branch GitHub URLs to PR branch

[zeitlinger]

Summary

• Add build_remap_args() to links.sh that generates lychee --remap flags to redirect /blob/{base}/ and /tree/{base}/ GitHub URLs to the PR branch
• Works in CI (using GITHUB_REPOSITORY, GITHUB_BASE_REF, GITHUB_HEAD_REF) and locally (parsing git remote URL and branch info)
• Supports fork PRs via PR_HEAD_REPO env var; skips silently when on the default branch or when repo info can't be determined

This eliminates the need for consuming repos to add boilerplate sed commands in their CI workflows to remap branch URLs during link checking (e.g., prometheus/client_java#1883).

Test plan

• Lint passes (mise run lint)
• On default branch (main): no remap args generated
• CI simulation (GITHUB_REPOSITORY/GITHUB_BASE_REF/GITHUB_HEAD_REF): correct remap args
• Fork CI simulation (with PR_HEAD_REPO): remaps to fork repo
• Head == base branch: no remap args generated

[Copilot]

Pull request overview

This PR adds automatic URL remapping functionality to the lint:links task to redirect base-branch GitHub URLs to the PR branch during link checking. This eliminates the need for consuming repositories to add manual sed commands in their CI workflows.

Changes:

• Added build_remap_args() function that generates --remap flags for lychee to redirect /blob/{base}/ and /tree/{base}/ URLs
• Modified run_lychee() to collect and pass remap arguments to lychee
• Supports both CI environments (via GITHUB_* env vars) and local execution (via git commands), with fork PR support via PR_HEAD_REPO

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[Copilot]

The remap pattern is missing regex anchors (^ and $). Without anchors, the pattern could match partial URLs incorrectly. The pattern should be anchored to match complete URLs: ^${base_url}/blob/${base_ref}/(.*)$

This is particularly important because lychee uses these patterns for regex matching, and without anchors, a URL like https://example.com/link-to-https://github.com/owner/repo/blob/main/file.md could be incorrectly matched and remapped.

Suggested change

	echo "${base_url}/blob/${base_ref}/(.*) ${head_url}/blob/${head_ref}/\$1"
	echo "--remap"
	echo "${base_url}/tree/${base_ref}/(.*) ${head_url}/tree/${head_ref}/\$1"
	echo "^${base_url}/blob/${base_ref}/(.*)\$ ${head_url}/blob/${head_ref}/\$1"
	echo "--remap"
	echo "^${base_url}/tree/${base_ref}/(.*)\$ ${head_url}/tree/${head_ref}/\$1"

[Copilot]

The remap pattern is missing regex anchors (^ and $). Without anchors, the pattern could match partial URLs incorrectly. The pattern should be anchored to match complete URLs: ^${base_url}/tree/${base_ref}/(.*)$

This is particularly important because lychee uses these patterns for regex matching, and without anchors, a URL like https://example.com/link-to-https://github.com/owner/repo/tree/main/path could be incorrectly matched and remapped.

Suggested change

	echo "${base_url}/blob/${base_ref}/(.*) ${head_url}/blob/${head_ref}/\$1"
	echo "--remap"
	echo "${base_url}/tree/${base_ref}/(.*) ${head_url}/tree/${head_ref}/\$1"
	echo "^${base_url}/blob/${base_ref}/(.*)$ ${head_url}/blob/${head_ref}/\$1"
	echo "--remap"
	echo "^${base_url}/tree/${base_ref}/(.*)$ ${head_url}/tree/${head_ref}/\$1"

[Copilot]

Branch names and repository names may contain special characters (like / in branch names for feature branches like feature/my-feature) that could break the regex pattern. These values should be escaped before being used in the regex pattern to prevent regex interpretation of special characters.

Consider using a regex escaping function or quoting mechanism to handle special characters like ., *, +, ?, [, ], {, }, (, ), ^, $, |, \, and /.