feat(recipe-upgrade): upstream release-notes registry + recipe-README read (recipe-maintainer parity)

Close the two gaps vs recipe-maintainer's recipe-upgrade-plan:
- Per-recipe release-notes registry at cc-ci-plan/upstream/<recipe>.md (discover the source repo +
  releases/changelog URL for each image once, persist+commit, reuse) — fetch release notes FROM those
  URLs instead of rediscovering ad-hoc each run. Format doc + cryptpad seed included.
- Explicitly read the recipe's README for shipped upgrade/migration notes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cc-ci-plan/upstream/cryptpad.md

@@ -0,0 +1,12 @@
+# Upstream sources — cryptpad
+
+| service | image | source repo | releases / changelog |
+|---------|-------|-------------|----------------------|
+| app     | cryptpad/cryptpad | https://github.com/cryptpad/cryptpad | https://github.com/cryptpad/cryptpad/releases |
+| web     | nginx             | https://github.com/nginx/nginx       | https://nginx.org/en/CHANGES |
+
+## Standing notes
+- `abra recipe upgrade` cannot parse the `version-YYYY.M.D` tag format of the cryptpad/cryptpad image,
+  so the app image is bumped by hand; the nginx sidecar is what `abra recipe upgrade` actually moves.
+- nginx is HTTP/1.1-only here (sidecar on :8083, `proxy_http_version 1.1`), so HTTP/2/3 CVE changes in
+  nginx releases generally don't affect this recipe — but still note them.