publish: a sync release with a non-v tag gets auto-generated notes instead of releasekit's #780

This was generated by AI during triage.

Found while reviewing #761.

Agent Brief

Category: bug
Summary: A sync release whose tag doesn't look like v1.2.3 gets GitHub's auto-generated notes instead of releasekit's notes.

Current behavior:
The publish stage matches release notes, and the changelog fallback, to a tag by parsing the tag string:

• It looks for a package name in the tag.
• Failing that, if there is exactly one notes entry, it uses that entry only when the tag matches ^v?\d+\.\d+\.\d+.

Sync notes are keyed monorepo (without mainPackage), which never appears in a tag. So a sync release tagged release-1.2.3 or app-v1.2.3 (any non-v versionPrefix, or a custom template once #732 makes sync honour tagTemplate) matches nothing. The GitHub Release is created with --generate-notes. LLM notes, human edits from the standing PR (#761) and the formatted changelog are all dropped silently.

Desired behavior:

• A release with one shared consumer tag (sync) gets the sole notes entry, or the sole changelog, whatever the tag looks like.
• With per-package tags, notes are matched through the version output's own mapping from update to tag (VersionPackageUpdate.tag → packageName) rather than by parsing tag strings, so any tagTemplate works.
• When notes exist but can't be matched to a tag, publish warns and names the tag, instead of silently falling back to auto-generated notes.

Key interfaces:

• The publish GitHub-release stage: its notes-for-tag and changelog-for-tag lookups.
• VersionOutput.tags, VersionPackageUpdate.tag and VersionOutput.strategy.

Acceptance criteria:

• Sync with tag release-1.2.3 and notes { monorepo: 'X' } creates the release with body X. The same holds for app-v1.2.3 and a bare 1.2.3.
• Per-package tags from a custom template, such as pkg-a/1.2.3, get their own package's notes.
• Existing v1.2.3 and @scope/pkg@v1.2.3 behaviour is unchanged.
• An unmatched tag with notes present logs a warning.

Out of scope:

• Tag naming itself (version: tag prefix and template defaults don't behave as configured #732).