Simplify author mention logic in release changelog generation:

• Always use .author.login from the commit API when building the changelog entries
• Remove fallback to .commit.author.name to avoid invalid @ mentions when only a name is available

Rationale: All commits land via PRs that are linked to GitHub users, so .author.login should always be available.

Test plan:

• Exercised on local branch by reading workflow; minimal risk as behavior matches expectation where author is a GH user.

If any commit ever lacks .author.login, the entry would produce an empty mention; but given our policy this should not occur.