ci: allow manual runs with workflow_dispatch

The workflow only triggered on push and pull_request, so a run that failed for
reasons outside the repo — runner down, toolchain changed underneath it —
could only be retried with an empty commit. That is a likelier situation with a
self-hosted runner than with hosted ones.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/ci.yaml

@@ -5,6 +5,10 @@ on:
     branches: [ master ]
   pull_request:
     branches: [ master ]
+  # Manual trigger. Worth having on a self-hosted runner: when a run fails for
+  # reasons outside the repo — the runner was down, a toolchain changed — this
+  # re-runs it without an empty commit to force one.
+  workflow_dispatch:
 
 # Both jobs target a self-hosted runner, by capability rather than by hostname.
 # See tool/setup_ci_runner.sh for how that runner is provisioned and why it is