Three of the four open PRs have no CI runs against their heads

[amadeobriones]

I've been running the open PRs locally against main (288146b) and noticed only #212 has any
check runs recorded:

PR	check-runs on head
#212	4 — two runs of validate-hacs + build, all green
#204	0
#184	0
#164	0

Checked two ways — repos/.../commits/{head_sha}/check-runs and each PR's statusCheckRollup —
and they agree. To be clear this is only about the Validate workflow: the Codex reviewer is
clearly working (its note on #164 about rotated plans is a good catch, and it reproduces). It's the
typecheck/test/build run that isn't happening.

validate.yml does declare on: pull_request, so the workflow does apply to these PRs — something
is stopping the runs rather than the trigger not matching. I can't see the repo's Actions settings,
so I won't guess at the cause.

Mentioning it because it's currently doing real work: #184 doesn't compile against main right
now, and Validate would have caught that in about ninety seconds without anyone reading a diff.

Happy to hear it's deliberate.