release-notes: take the paragraph from the tag object, never a commit

[tinic]

beta7's published notes carried the #113 squash commit's body instead of the annotated tag's paragraph. They had to be corrected by hand.

Cause. For a tag push, actions/checkout leaves refs/tags/<tag> pointing at the commit, not the annotated tag object. On such a ref, git tag -l --format='%(contents:body)' returns the commit message, which tools/release-notes.sh treated as the tag's body. The script's comment assumed a lightweight tag yields nothing; in fact it yields the commit's body.

Fix

• tools/release-notes.sh reads the paragraph only when refs/tags/$tag is a tag object. A ref to a commit gets no paragraph, which is the documented lightweight-tag behaviour.
• release.yml fetches the annotated tag object (git fetch --force origin +refs/tags/$tag:refs/tags/$tag) before rendering, so the real paragraph is used.

Checked on the v1.0.0-beta7 tree

• With the annotated tag, the notes open with its four bullets.
• With the tag rewritten as a commit ref (the CI case), the notes have no paragraph instead of the commit body.
• After the fetch, the tag is a tag object again.

Not addressed: the notes still take the CHANGELOG section from the tag's tree. A rerun on v1.0.0-beta7 therefore renders the beta7 section as it stood at 7fd931d, without the later #117/#118 corrections.

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