Update changelog bundle-amend command to support CDN changelogs

[lcawl]

Summary

Enhance the --add and --remove options in the docs-builder changelog bundle-amend command so they work with files in S3/CDN not just local files.

Background

changelog bundle already treats the CDN as the source of truth for changelog YAML when bundle.repo resolves. --files accepts CDN paths (or any path reduced to a basename) and does not require those files on disk. That matches how teams are expected to work: upload entries, bundle from the pool, then delete local YAML (changelog remove) because bundles are self-contained.

changelog bundle-amend never joined that model. --add and --remove call File.Exists and read disk only. There is no CDN fetch and no --force-local. The result is a broken post-publish workflow for the common case:

• Remove an entry from an existing bundle when the changelog lives only on the CDN (or was already cleaned up locally). Today you must invent a dummy file whose name matches file.name. The dummy’s checksum will not match, so you also need --force. That is ceremony, not provenance: --force already means “match by name.”
• Add a changelog that exists only on the CDN (uploaded, never checked out, or private-repo copy that exists in S3 with scrubbed public PR/issue refs). --add cannot embed that YAML unless you download it by hand first. bundle --files already solves this for new bundles; amend cannot fix a published bundle the same way.

Implementation details

changelog bundle-amend now uses the same CDN vs local sourcing gate as changelog bundle --files. The changelog-tool skill is unchanged, and nothing was committed.

Behavior

• CDN by default when bundle.repo or the parent bundle’s repo resolves.
• --add / --remove match by file name (CDN paths like /changelog/elastic/kibana/main/247279.yaml work; no local file required).
• --force-local (and bundle.use_local_changelogs) still reads disk.
• --force --remove 300.yaml excludes inferred git-ref entries without a dummy file.
• CdnChangelogEntryFetcher.FetchNamedAsync downloads only the requested names.

docs-actions: no bundle-amend usage; no changes there.

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

[Mpdreamz]

Reviewed manually (AI bot review is blocked today by the daily AI-credits guardrail). Verified: builds clean, dotnet test tests/Elastic.Changelog.Tests passes (1043/1043), CDN vs local sourcing gate is shared via the new ChangelogEntrySourcing helper, and the add/remove/force/force-local paths are covered by new tests including CDN-miss, checksum-mismatch, and force-local isolation cases.