docs: add PR merge readiness checklist

[Thump604]

Addresses #482.

This adds an in-repo PR template checklist for the process failures identified in the self-merge audit:

• authors should not merge their own substantive PRs
• latest CI for the exact head commit should be green before merge
• post-review rebases/conflict fixes need a fresh green CI run
• actionable review comments should be resolved or linked to follow-up issues before merge
• intentional follow-up debt should be visible in the PR body

This does not replace branch protection or repository admin settings. It is a lightweight visible guardrail inside the repository so contributors and maintainers see the expected merge-readiness checks on every PR.

Validation:

• docs/template-only change
• git diff --check

[janhilgard]

Clean docs-only PR. The checklist covers the right failure modes from the self-merge audit — particularly "latest CI for the exact head commit" (not a stale run) and "fresh CI after post-review rebase" which are easy to miss in practice.

The "I am not merging my own substantive PR" checkbox is a good social guardrail even without branch protection enforcement.

LGTM.