[rumblefrog]

Summary

Fixes #1474.

The Anonymous telemetry help paragraph in
?p=admin&c=settings&section=features ships a "See the full payload"
link pointing at https://sbpp.github.io/updating/1.8-to-2.0/#telemetry,
which 404s. Two things are wrong:

1. Slug 1.8-to-2.0 — Astro derives slugs from filenames, and the
   docs source file on disk is docs/src/content/docs/updating/1-8-to-2-0.mdx
   (dots become dashes). The deployed URL is /updating/1-8-to-2-0/.
2. Anchor #telemetry — the heading in that file is ## Anonymous telemetry, which Starlight auto-slugs to #anonymous-telemetry.
   #telemetry matches no heading.

Three sibling sites carried the same broken URL or the matching
source-file path drift, all swept here:

• web/themes/default/page_admin_settings_features.tpl — the actual
  user-reported broken link.
• AGENTS.md "Build / extend the anonymous opt-out daily telemetry
  payload" row pointed at the wrong source file path
  (docs/src/content/docs/updating/1.8-to-2.0.mdx, which doesn't exist).
• CHANGELOG.md 2.0.0 Privacy subsection — same broken URL.

Regression guard

New web/tests/integration/DocsUpgradeLinkRegressionTest.php pins
four contracts:

1. testCanonicalDocsFileExistsAtFixedSlugPath — the docs source
   file at 1-8-to-2-0.mdx exists. A rename silently re-breaks every
   panel-side link pointing at the slug; pinning the canonical
   filename here makes a future "let's clean up the slug" PR fail
   the gate first.
2. testCanonicalDocsFileCarriesTheAnonymousTelemetryHeading —
   that file carries the ## Anonymous telemetry heading. A rename
   to ## Telemetry would silently 404 every deep-link to
   #anonymous-telemetry.
3. testNoPanelOrDocSourcesUseTheBrokenUpgradeSlug — no file
   under the documented scan roots (templates, page handlers,
   includes, scripts, API handlers, AGENTS.md, CHANGELOG.md)
   carries the literal 1.8-to-2.0 substring. Catches both broken
   URLs (/updating/1.8-to-2.0/) and broken source-file path
   references (docs/src/content/docs/updating/1.8-to-2.0.mdx)
   with one assertion.
4. testTelemetryHelpLinkUsesCorrectUrlAndAnchor — the specific
   template line uses the fully corrected URL. A half-fix that gets
   one half right but the other wrong (slug right, anchor wrong, or
   vice-versa) still fails.

Pure file scanning — no DB / session / Smarty bring-up. Extends
PHPUnit\Framework\TestCase directly (same shape as
DeadJsCallSitesTest, IframeChromeAntiFoucBootloaderTest,
ButtonClassChainTest).

Dev-stack symmetry

docker-compose.yml now mounts ./docs, AGENTS.md, and
CHANGELOG.md read-only into the web container. This mirrors the
existing ./docker:/var/www/html/docker:ro pattern (added for
ProdApacheConfigTest so the dev container could read production
configs the runtime panel never touches). Without these mounts the
new gate would fail under ./sbpp.sh test locally even though CI
passes — CI's actions/checkout@v4 sees the full repo on the
runner. docker/README.md updated to describe the new mounts.

Test plan

• ./sbpp.sh phpstan — OK (252 files, no errors)
• ./sbpp.sh test --filter=DocsUpgradeLinkRegressionTest — OK (4 tests, 13 assertions)
• ./sbpp.sh test — 1075 tests pass; 6 pre-existing errors in PluginVersionResolveTest (dev container doesn't bind-mount game/, same on main)
• ./sbpp.sh ts-check — clean
• ./sbpp.sh composer api-contract — no diff
• Negative control: reintroduced the broken slug in the template and confirmed the test fails (2 of 4 assertions caught it — testTelemetryHelpLinkUsesCorrectUrlAndAnchor AND testNoPanelOrDocSourcesUseTheBrokenUpgradeSlug)

I have read the CLA Document and I hereby sign the CLA