ci: revert to squash-merge

[plttn]

PR #91 merging via a real merge commit produced two identical changelog entries in the pending release PR (#84): GitHub embeds the PR title verbatim in the merge commit body, and since PR titles here often already read as conventional commits, release-please parsed both the merge commit and the original branch commit as separate changes (googleapis/release-please#2476).

#90 moved off squash-merge specifically to avoid jj dangling-commit buildup after merges. Tested that directly in a throwaway git+jj repo: simulated a real GitHub squash-merge (new commit on main, source branch deleted) and ran jj git fetch. The orphaned commit and its bookmark were both cleaned up automatically — git.abandon-unreachable-commits (on by default) plus this repo's delete_branch_on_merge: true already handle it. The premise for avoiding squash doesn't hold with the current jj version and repo settings.

This reverts #90's CI/docs changes (restoring the pr-title check and squash-based wording in CONTRIBUTING.md, AGENTS.md, and the PR template) and switches the repo's merge button settings back to squash-only, with the squash commit message set to just the PR title. No merge commit is ever created going forward, so there's nothing for release-please to double-parse.

Test plan

• mise run lint passes
• Repo settings confirmed: allow_squash_merge=true, allow_merge_commit=false, squash_merge_commit_title=PR_TITLE, squash_merge_commit_message=BLANK
• After this PR squash-merges, confirm the resulting commit on main is a single commit whose message is exactly this PR's title