[Release notes] Cutover to product-version changelog note registry

[lcawl]

Summary

This PR is the fourth and final phase of fixes for changelog note registry issues.
When it is merged, we can delete leftover notes-{version}.json objects from S3/CDN.
{changelog} :cdn: pages no longer read them.

Background

Relates to #4090, #4103

Per #4103 (comment), since the related issues touch the same areas of the code and have a significant, overlapping impact, this PR is part of a multi-PR phased approach.

Details

CdnChangelogFetcher only loads:

1. bundle/{product}/registry.json
2. The YAML files that registry lists (cloud-4.2.0.yaml, .amend-N, .amend-notes.yaml)

src/Elastic.Markdown never references notes-. Notes indexes are only used by changelog bundle.

Do not delete bundle/{product}/*.yaml or bundle/{product}/registry.json. Those are what the pages render.

This cutover:

• Stops writing notes-{version}.json
• Deletes leftover version-only keys on the next notes reconcile
• Drops the FetchNotesAsync 404 fallback; changelog bundle only GETs notes-{product}-{version}.json

A note upload (or --skip-etag-check on an existing note) is enough to trigger leftover version-only deletes. Manual S3 deletes of those objects are also safe for directive pages.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor Grok 4.6

[github-actions]

Docs preview (local build)

Handbook preview: https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/4124/

Sample changed pages:

• docs/cli/changelog/cmd-backfill.md
• docs/cli/changelog/cmd-note.md
• docs/development/changelog-bundle-registry.md

[github-actions]

Approved.

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