Read a frozen changelog whose title is not the configured one

[cjbarth]

The bug

_readChangelogSections found the first release by stripping an exact match of
template.changelogTitle from the top of the file. Anything else up there — a funding
banner added to the title, or a title whose wording changed since the file was last
written — did not match, so the preamble stayed attached to the release below it. That
release's section was then keyed off the preamble's first line, which has no version in
it, so the section was dropped from the map frozenBefore copies from, and the next
gren changelog --generate wrote the file without it.

It fails silently, and only when the newest section in the file is old enough for
frozenBefore to freeze it — which is exactly the case where nothing regenerates it.

The fix

The title is now whatever precedes the first release heading, so what is read back
depends on the configuration rather than on what the file being replaced happens to say.
The exact-title strip stays as the fast path for a release template that is not a
heading. The heading pattern the section split already used is now shared with the
preamble scan, so the two cannot drift apart — this bug was the two disagreeing about
where a release starts.

Verification

Both tests were written first and the first one failed for the reported reason
(expected undefined to equal '## v2.0.0 (2020-01-15)...').

Run over this repository's own history — 14 releases, dataSource: prs, frozenBefore —
writing to a scratch file in a throwaway clone, never over a real CHANGELOG.md:

• No banner, unchanged title: output is byte-identical before and after. No diff.

• Banner in changelogTitle, file on disk still carrying the old title (the first run
  after adding a banner), with frozenBefore set late enough to freeze every release:
  master wrote 45 release headings, this branch wrote 46. The diff is the whole of
  v5.1.1, silently missing on master:

  +## v5.1.1 (2026-09-16)
  +
  +### Bug Fixes:
  +
  +- [#17](...) Gather commits no release contains under an Unreleased heading
  +
  +---

  The release list this branch writes matches the committed CHANGELOG.md exactly.

npm test 133 passing; env -u GREN_GITHUB_TOKEN npx mocha 94 passing, 39 pending;
npm run lint clean apart from the pre-existing mocha/no-pending-tests warning in
test/Gren.spec.js. markdownlint-cli2 reports the same three pre-existing errors on the
generated file as on the committed CHANGELOG.md, and none that are new.

🤖 Generated with Claude Code