Everything-is-an-app: non-blocking CI comment on vendor values.schema.json drift

## Parent

linode/apl-core#3515

## What to build

A non-blocking CI safety net that flags vendor-schema drift when a chart version is bumped, so maintainers can note likely-breaking changes without owning an ongoing migration mechanism.

End-to-end behavior:
- When a PR changes a chart version in `charts/dependencies.yaml`, CI diffs the old vs. new vendor `values.schema.json` (for charts that ship one) and posts a summary comment on the PR.
- The check is **informational only** — never a merge gate — and does not attempt to auto-migrate anything.
- Charts without a vendor schema are skipped.

## Acceptance criteria

- [ ] On a PR that bumps a chart version with a vendor schema, CI posts a comment summarizing the `values.schema.json` diff.
- [ ] The check never blocks merge and produces no failure status.
- [ ] Charts without a vendor schema are ignored.

## Blocked by

None - can start immediately.

Start with charts/dependencies.yaml and the repository's existing CI configuration; trace how chart-version changes are detected. Compare the old and new vendor values.schema.json for charts that provide one, then verify that a pull request receives an informational comment, no failure status, and skips charts without schemas.

Written by the indexing model from the issue text.