Add lychee link checker config and CI workflow

Adds a lychee configuration and a Links GitHub Actions workflow so that
stale or redirecting links are caught automatically going forward. The
checker runs on push, pull request, and weekly to catch external link
rot. max_redirects is 0 so links that have moved are surfaced and can be
updated to their canonical destination.

Part of STF-557.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: oschwald

.github/workflows/links.yml

@@ -0,0 +1,32 @@
+name: Links
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: "0 13 * * 1" # weekly, to catch external link rot without a commit
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  linkChecker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+
+      - name: Setup mise
+        uses: jdx/mise-action@6d1e696aa24c1aa1bcc1adea0212707c71ab78a8 # v3.6.1
+        with:
+          install: false
+
+      # Install only lychee (not the repo's full toolchain) and run the check.
+      - name: Check links
+        env:
+          MISE_AUTO_INSTALL: "false"
+        run: |
+          mise install lychee
+          mise run check-links

.gitignore

@@ -20,3 +20,4 @@ pom.xml.versionsBackup
 target
 reports
 Test.java
+.lycheecache

lychee.toml

@@ -0,0 +1,64 @@
+# Lychee link checker configuration
+# https://lychee.cli.rs/#/usage/config
+#
+# Run locally with:
+#   lychee './**/*.md' './src/**/*.java' './pom.xml'
+
+# Include URL fragments in checks
+include_fragments = true
+
+# Don't allow any redirects, so links that have moved are surfaced and updated
+# to their canonical destination.
+max_redirects = 0
+
+# Accept these HTTP status codes
+# 100-103: Informational responses
+# 200-299: Success responses
+# 403: Forbidden (some sites use this for rate limiting)
+# 429: Too Many Requests
+# 500-599: Server errors (temporary issues shouldn't fail CI)
+# 999: LinkedIn's custom status code
+accept = ["100..=103", "200..=299", "403", "429", "500..=599", "999"]
+
+# Exclude URL patterns from checking (treated as regular expressions)
+exclude = [
+    '^file://',
+    # Live / auth-gated endpoints that appear as string literals or require login
+    '^https://geoip\.maxmind\.com',
+    '^https://geolite\.info',
+    '^https://minfraud\.maxmind\.com',
+    '^https://sandbox\.maxmind\.com',
+    '^https://updates\.maxmind\.com',
+    '^https://www\.maxmind\.com/en/accounts/',
+    'https://www\.maxmind\.com/en/account/login',
+    # XML namespace identifiers in pom.xml (not real links)
+    '^http://www\.w3\.org/',
+    '^http://maven\.apache\.org/',
+    '^https://maven\.apache\.org/xsd/',
+    '^http://java\.sun\.com/',
+    '^http://schemas\.',
+    # Maven property placeholder in a build-time download URL (not a real link)
+    'japicmp\.baselineVersion',
+    # Placeholders / local
+    '^https?://example\.(com|org|net)',
+    '^http://localhost',
+    '127\.0\.0\.1',
+]
+
+# Exclude file paths from getting checked (treated as regular expressions)
+exclude_path = [
+    '(^|/)node_modules/',
+    '(^|/)target/',
+    '(^|/)\.git/',
+    # Test fixtures (MaxMind-DB submodule) contain example URLs, not ours
+    '(^|/)src/test/resources/',
+    # Changelog: historical entries are preserved as-is, not rewritten
+    '(^|/)CHANGELOG\.md$',
+]
+
+# Cache results for 1 day to speed up repeated checks
+cache = true
+max_cache_age = "1d"
+
+# Skip missing input files instead of erroring
+skip_missing = true

mise.lock

@@ -9,6 +9,7 @@ disable_backends = [
 [tools]
 hugo = "latest"
 java = "latest"
+lychee = "0.23.0"
 maven = "latest"
 
 [tasks.build-docs]
@@ -19,6 +20,10 @@ run = "hugo --source docs --minify"
 description = "Serve the docs site locally with Hugo dev server"
 run = "hugo server --source docs"
 
+[tasks.check-links]
+description = "Check links with lychee"
+run = "lychee --no-progress './**/*.md' './src/**/*.java' './pom.xml'"
+
 [hooks]
 enter = "mise install --quiet --locked"