feat(links): add GitHub URL remaps for line-number and fragment anchors

[zeitlinger]

Summary

• Add ordered remaps in build_remap_args() for GitHub /blob/ and /tree/ URLs with line-number anchors (#L123) and other fragments (#section)
• Line-number anchors are stripped (they're JS-rendered and unverifiable by lychee); other fragments are remapped to raw.githubusercontent.com as a workaround for lychee#1729
• Consuming repos no longer need custom remap rules in lychee.toml for these patterns
• Add LYCHEE_SKIP_GITHUB_REMAPS=true opt-out env var

Context

GitHub blob URLs with line-number anchors (#L6) are JavaScript-rendered and can't be verified by lychee (which fetches static HTML). When the existing branch remap rewrites blob/main/ → blob/<pr-branch>/, these URLs fail fragment checking.

Since lychee uses first-match-wins (no chaining), config file remaps never run after flint's CLI --remap matches. The fix belongs here in build_remap_args().

Remap order (first-match-wins)

For /blob/ URLs:

1. Line-number anchors (#L123): strip fragment, check file on base branch
2. Other fragment URLs (#section): remap to raw.githubusercontent.com + head branch
3. Non-fragment URLs: branch-remap only (existing behavior)

For /tree/ URLs: rules 1 and 3 only (no raw remap needed).

Test plan

• Run mise run lint:links --full in a consuming repo (e.g. grafana-opentelemetry-java) with a #L6 link
• Verify the link passes without custom remap rules in lychee.toml
• CI passes

[Copilot]

Pull request overview

This PR updates the lychee link-lint task to better handle GitHub /blob/ and /tree/ URLs that include fragments, especially line-number anchors that lychee can’t verify reliably on GitHub’s JS-rendered pages.

Changes:

• Add ordered GitHub --remap rules for /blob/ and /tree/ URLs to handle line-number anchors and other fragments.
• Strip #L123 fragments (and introduce a raw-content remap for other fragments) to reduce false negatives during fragment checking.
• Add LYCHEE_SKIP_GITHUB_REMAPS=true to opt out of GitHub-specific CLI remaps emitted by the task.

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[Copilot]

The line-anchor remap keeps the URL on the base branch (${base_ref}), which contradicts the function’s stated goal of making base-branch links resolve on the PR/head branch. This will make PRs fail when they add a new file (or rename/move one) and link to it via .../blob/${base_ref}/...#L123, because the stripped URL will 404 on the base branch even though it exists on the head branch. Consider stripping the fragment but still rewriting to the head repo/ref (similar to the non-fragment rule).

[Copilot]

Same issue as the /blob/ rule: the /tree/ line-anchor remap strips the fragment but keeps the URL on the base branch. If a PR introduces a new directory/link target (or moves it) and uses a .../tree/${base_ref}/...#L123 link, the remapped URL will be checked against the base branch and can 404 even though it exists on the head branch. Consider stripping the fragment while still remapping to ${head_url}/tree/${head_ref}/....

Suggested change

	# 1. Line-number anchors: strip fragment (unlikely for tree but consistent)
	echo "--remap"
	echo "^${base_url}/tree/${base_ref}/(.*?)#L[0-9]+\$ ${base_url}/tree/${base_ref}/\$1"
	# 1. Line-number anchors: strip fragment, remap to head branch
	echo "--remap"
	echo "^${base_url}/tree/${base_ref}/(.*?)#L[0-9]+\$ ${head_url}/tree/${head_ref}/\$1"

[Copilot]

The comment says raw.githubusercontent.com “lets lychee verify the fragment in the raw HTML”, but raw.githubusercontent.com serves the raw file contents (not rendered HTML). Consider rewording to avoid implying GitHub is returning HTML here (e.g., “verify fragments against the raw content”, or describing the specific behavior lychee uses for fragment checks).

Suggested change

	# which lets lychee verify the fragment in the raw HTML (workaround for
	# which lets lychee verify the fragment against the raw file content (workaround for

[Copilot]

LYCHEE_SKIP_GITHUB_REMAPS=true currently only disables the GitHub remaps emitted by build_remap_args(), but this repo’s .github/config/lychee.toml also defines a GitHub remap. Consider clarifying the comment to avoid implying this env var disables all GitHub remapping behavior in lychee, or remove/move the config remap if the intent is to centralize remaps here.

Suggested change

	# Set LYCHEE_SKIP_GITHUB_REMAPS=true to disable all GitHub-specific remaps.
	# Set LYCHEE_SKIP_GITHUB_REMAPS=true to disable the GitHub-specific remaps
	# emitted by this function (does not affect remaps defined in lychee config).