Rework the PR template and check that it is filled in

[amankrx]

What and why

The PR template asks for things CI already proves and not much a reviewer actually needs, so it gets filled in as ceremony. Measured across the last 33 merged PRs that used it:

• 29/33 tick "bazel test //... passes locally". Nobody runs the full suite before every PR, so the box carries no information.
• 17/33 tick every box, including all four mutually exclusive "Type of change" options.
• 15/33 leave "Please delete options that aren't relevant" in the body.
• 14/33 have an effectively empty "How Has This Been Tested?", which is the one section a reviewer cannot get from the diff.
• 6/33 leave "Fixes # (issue)" verbatim.

Several of those are mine. This replaces it with three sections that each ask for something only the author knows: what problem this solves, how it was actually verified, and what breaks if it is wrong. All guidance moved into HTML comments so it cannot leak into the permanent record.

The checklist is gone. bazel test //..., formatting and lints are already CI jobs and pre-commit hooks, which enforce them properly rather than on the honour system.

How this was verified

Ran the checker against real inputs: a filled-in body passes, the unedited template fails on all three sections, a body with no headings fails, an empty body fails, and a body with "n/a" under a heading fails on that heading only. Also ran it against #2683, an existing PR using the old template, and confirmed it reports the headings as missing rather than crashing.

Workflow YAML parses and the triggers resolve as intended. I could not exercise the workflow end to end, since that only happens once it is on the default branch.

Risk

This is a process change, so it needs agreement rather than just review.

The check fails PRs whose description is thin, which is the point, but two details keep it from being hostile:

• It skips bots and honours a skip-pr-template label, which a maintainer will need to create.
• It comments saying exactly which section is short and re-runs on edited, so it goes green as soon as the author fixes it. A red X with no explanation is how you lose a drive-by contributor.

Open PRs written against the old template will fail this check the next time they are edited or pushed to. The label is the escape hatch, or hold the merge until the queue drains.

It runs on pull_request_target so it can comment on fork PRs, where a first-time contributor most needs the hint. Nothing from the PR is checked out or executed: the checkout is the base branch, and the description is only ever read from a file, never interpolated into a shell command.

No new third-party action; it reuses the actions/checkout pin already trusted in this repo and the preinstalled gh CLI.

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[MarcusSorealheis]

LGTM

[modernmedici]

Looks good, just adjusted the wording in one place to make sense.