Let CI be started by hand

CI triggers on a push to main and on pull requests, and on nothing else.
That left no way to put a check on 7a1e5ee -- the commit production is
running -- after GitHub's Actions outage on 2026-08-26 orphaned every run
created during it.

Those runs are not merely slow. GitHub accepted them, allocated zero
jobs, and left them in a state its own API contradicts itself about:

  gh run rerun   ->  "cannot be rerun; This workflow is already running"
  gh run cancel  ->  "Cannot cancel a workflow run that is completed"
  gh api         ->  status=queued, conclusion=null, jobs=0

Neither recoverable nor clearable, and with no manual trigger the only
remaining option would have been an empty commit pushed to main to move
the ref -- which is both a junk commit and against how changes land here.

workflow_dispatch also covers the ordinary case of wanting a check on a
commit that predates a CI change.

.github/workflows/ci.yml

@@ -3,6 +3,14 @@ on:
   push:
     branches: [main]
   pull_request:
+  # Lets CI be run by hand against any ref, including a specific commit.
+  # Without this there is no way to re-run a check that never started: a run
+  # GitHub queues and then orphans -- as it did to every run created during the
+  # Actions outage on 2026-08-26 -- can be neither rerun ("already running")
+  # nor cancelled ("already completed"), and the workflow has no other trigger
+  # to reach for. Useful too for putting a check on a commit that predates a CI
+  # change, without pushing an empty commit to move it.
+  workflow_dispatch:
 jobs:
   build:
     runs-on: ubuntu-latest