feat(changelog): resolve Lisa's review on #789 — versions, amend reconciler, backport rule

[Mpdreamz]

Summary

Resolves the four points Lisa raised on elastic/docs-eng-team#789.

• target obsoleted on entry/note schema. versions (note-only, plural list) replaces it. Nothing writes target any more; readers still accept it for existing pool objects. BundledProduct.Target (bundle schema) is unchanged and tracked separately in elastic/docs-eng-team#790.
• Notes pass through the pool — always did; updated decision 4 wording to reflect this.
• NoteAmendReconciler: Lambda auto-generates .amend-notes.yaml sidecars for notes that arrive after their release bundle has shipped. One aggregate sidecar per parent bundle, rebuilt from state on every reconcile (idempotent). bundle_seq on NoteIndexEntry (0/1/2) reports shipped state cheaply.
• Backport collision rule: same-leaf note on multiple branches at one version → prefer main/master, warn on discards, ordinal-first otherwise.

What changed

Area	Change
Schema	ProductInfoDto.Target → [Obsolete]; ProductInfoDto.Versions added; NoteIndexEntry drops branch, adds bundle_seq; schema_version: 1
Authoring	changelog note writes versions; changelog add errors if a version is supplied in --products
Reconciler	NotesIndexKey param renamed target→version; conditional writes + retry; NoteAmendReconciler (new)
Bundling	AmendDocumentBuilder extracted (pure); BundleAmendMerger / ChangelogBundleAmendService widen .amend-notes as reserved suffix; backport dedup in MergeNotesAsync
Docs	New cmd-note.md; updated cmd-add.md, cmd-bundle-amend.md, changelog-fields.md, changelog-bundle-registry.md; CLI schema regenerated
Follow-ups	elastic/docs-eng-team#790 #791 #792 #793 #794 filed

Test plan

• dotnet build — 0 errors
• dotnet test tests/Elastic.Changelog.Tests/ — all passing (1009 tests)
• dotnet test tests/Elastic.Documentation.Configuration.Tests/ — all passing (681 tests)
• ./build.sh lint — clean (pre-push hook passed)
• New NoteAmendReconcilerTests (8 tests): late note → sidecar; already shipped → no sidecar, seq=1; human amend → no sidecar; no file annotations → skipped; delete path; idempotent redelivery; no matching bundle; no products in bundle tree
• New BundleCdnSourcingTests backport tests (3 tests): main wins, ordinal pick, checksum dedup still works
• Reply comment posted on elastic/docs-eng-team#789; decision list updated

🤖 Generated with Claude Code

[Mpdreamz]

Terminology can do a re-eval for sure

roughly:

release -> bundle
entry -> bundle entry

entry exists of

1 or more <pr>.yaml changelog files.
or 1 note-<name>.yaml.

A release entry is a single entry in the release that will be displayed as a single unit.