ci: fetch full history before amending the release commit

[xiaoyijun]

Summary

Fixes the bug that just destroyed release PR #245. The sign-release-branch job from #268 used the default shallow checkout (fetch-depth: 1): git commit --amend in a clone whose parent objects are behind the shallow boundary rewrote the release commit into a parentless orphan containing the whole repo tree (181 files, hence the size/xl relabel). Force-pushing that orphan left the release branch with no common ancestor with master, so GitHub auto-closed #245 as unmergeable — attributed to silverhand-bot, whose PAT pushed it.

Two changes:

• fetch-depth: 0 on the checkout, so the amend keeps the commit's parent;
• a post-amend guard: if the amended commit has an empty %P, abort instead of pushing — a history rewrite must never reach the branch again, whatever the cause.

Recovery

No manual branch surgery needed — merging this PR is the recovery: the master push makes release-please notice its PR is closed, force-push a fresh, correctly-parented release commit over the orphan, and open a new release: 3.0.0-beta PR; that push then triggers the fixed sign job, which amends it with the bot's GPG key. The new release PR should come up green and Verified, ready to merge.

Testing

• Reproduced the failure mode from the evidence: c693c58 has parents: [] and the full tree as its diff, exactly what a shallow amend produces.
• The guard condition (git log -1 --format=%P empty on orphan, non-empty otherwise) is the direct negation of the corruption.

Checklist

• .changeset (N/A — release-please)
• unit tests (N/A — workflow change)
• integration tests (N/A)
• necessary KDoc comments (N/A)

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