ci: bump checkout to v5 and add all-checks-passed gate

[jnasbyupgrade]

Small CI-hygiene changes, independent of the larger test-capabilities work.

• actions/checkout@v4 → v5 — v4 emits a Node 20 deprecation warning on current runners.
• all-checks-passed aggregation job — one stable required-status-check name for branch protection, instead of the matrix-dependent 🐘 PostgreSQL <n> names. Passes if every needed job succeeded or was skipped; fails if any failed/cancelled.
• Scope push trigger to master — on: [push, pull_request] double-runs CI on every commit to a PR branch (once as push, once as pull_request) for the same SHA; a flaky run could then show red next to an identical green one. PR branches now run only via pull_request; master runs post-merge via push.

No change to what's tested (still pg-build-test across the PG 9.3–17 matrix). Follow-up PRs will add the pg_upgrade and pg_tle jobs.

🤖 Generated with Claude Code

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