fix(release): fold the beta changelog into the release that supersedes it

[derrickmehaffy]

You're right — as it stands the 5.1.0 release PR reads as though almost nothing
shipped:

## [5.1.0]        Bug Fixes, Documentation, Chores        →  3 entries
## [5.1.0-beta]   Features, Bug Fixes, Performance, …     → 33 entries

release-please writes one section per release, and a prerelease is a release.
So everything that actually went into 5.1.0 sits under a heading for a version
almost nobody installed. Anyone upgrading 5.0.1 → 5.1.0 wants one list.

Fix

Prerelease sections are folded into the stable release sharing their
major.minor.patch, with subsections restored to the order
changelog-sections declares — merging appends whole groups, so without that a
reader meets Chores before Features. The compare link widens to where
the earliest prerelease began, since the stable release now spans it:

compare/v5.1.0-beta...v5.1.0   →   compare/v5.0.0...v5.1.0

Only the topmost section is considered — the release being prepared. My
first version folded every stable release and rewrote history: it merged the
2022 4.0.0-alpha.0 and 4.0.0-alpha.1 sections into 4.0.0, losing which
alpha introduced what, in releases that shipped years ago.

A second bug, found while wiring it up

The step's RELEASE_BRANCH was hardcoded to release-please--branches--main.
That stopped being the release branch when #216 removed
separate-pull-requests: false and the strategy started naming its own —
release-please--branches--main--components--monorepo.

A hardcoded name doesn't fail, it silently skips: the git ls-remote guard
simply found nothing, so the changelog was quietly going untidied. It is now
read from the open release pull request, so it survives future config changes.

Verified against the real release branch

resolved branch: release-please--branches--main--components--monorepo
folded 1 prerelease section into its stable release
no duplicates found
5.1.0 entries: 36        (3 + 33, nothing lost)

• subsections in canonical order: Features, Bug Fixes, Performance, Refactoring, Documentation, Chores
• the two 4.0.0-alpha sections untouched
• a second run reports no prerelease sections to fold — idempotent
• the deduplicator finds nothing left afterwards, and max consecutive blank lines is 2

Note

This is general, not a one-off patch — but it should rarely fire again. Betas
are now manual dispatches that publish to npm without a tag or a GitHub Release,
so they no longer produce changelog sections at all.

🤖 Generated with Claude Code