Fix stale changelog registry

[lcawl]

This PR must be merged after #3801 since it uses that PR as its base.

Context

changelog bundle CDN sourcing today does this:

flowchart LR
  bundleCmd["changelog bundle"] --> registry["GET changelog/org/repo/branch/registry.json"]
  registry --> listed["Download every listed YAML"]
  listed --> match["Filter by optional YAML prs/issues/products"]

Loading

After #3760, nothing writes changelog/{org}/{repo}/{branch}/registry.json. The scrubber only reconciles bundle/{product}/registry.json. Uploaded YAML is on S3/CDN; the pool index is frozen.

This means that changelogs that I've uploaded recently to the CDN are not findable by the docs-builder changelog bundle command because they're not reflected in the registry.

Implementation details

What changed in docs-builder

• The scrubber now owns both listings. YAML (and registry-key) events reconcile changelog/{org}/{repo}/{branch}/registry.json from the public YAML listing. target is always null for the pool. Client JSON is not copied.
• --prs (local and CDN) uses the same join as git-ref: leading filename digits or YAML prs:. Empty prs: still matches 12345.yaml.
• CDN --files / a path list GETs those basenames only (no registry.json). A missing name still fails the run.
• CDN --all and product-only filters error and point at --force-local. Local --all is unchanged. Git-ref infer is unchanged.

After merge, Kibana’s stale listing only heals once the scrubber Lambda is deployed. CLI-only is not enough. A --skip-etag-check upload (or the next YAML event) after that deploy rewrites changelog/elastic/kibana/main/registry.json.

Behavior to be aware of: a CDN monthly/product-only bundle (no PR/file list), like the cloud products: profile with only a date argument, now errors until phase 3. Use --force-local for those runs (this should be fine since we've now removed those profiles).
Kibana's serverless-release … prs.txt is --prs and is the path this fixes.