fix: read the version from release-please headings without a compare link

[non7top]

When a repo has no earlier release tag, release-please writes the changelog heading as ## 0.1.0 (date) with no brackets or compare link. The action only matched ## [X.Y.Z], so it reported no version (seen at non7top/sigstore-shell#9 (comment)). Both dry-run greps now accept the unbracketed form.

The warning for a PR that changes an already-pending version now says it overwrites the proposed version (v1.0.0 becomes v0.1.0 (minor)).

Not run end to end; only the edited echo line was syntax-checked.

[github-actions]

🤖 release-please PREVIEW

What merging this PR into master would trigger:

A release is already pending on master (v1.7.0, minor). This PR adds to it without changing the version. Entries marked already pending are not from this PR.

_{The bump is the highest of everything merged since v1.6.0, so other open PRs can move it too.}

🤖 I will create a release beep boop

1.7.0 (2026-10-08)

Features

• mark already-pending entries when a PR also adds to the pending release (e7d576f) — already pending
• mark already-pending entries when a PR also adds to the pending release (9cf1b69) — already pending
• warn when a PR changes the bump of an already-pending release (f161202) — already pending

Bug Fixes

• read the version from release-please headings without a compare link (the merge commit GitHub would create for this PR)
• read the version from release-please headings without a compare link (288f408)

---

This PR was generated with Release Please. See documentation.

_{Two entries, one change: release-please reads both the commit on this branch and the merge commit GitHub will create, whose body is this PR title. Both really do land in the published changelog. Only the merge commit has no SHA to cite, because it does not exist yet.}

_{Predicted from 288f408 merged into master at e7d576f, with the PR title read as fix: read the version from release-please headings without a compare link · Full run}