ci: allow manual runs via workflow_dispatch

Lets CI be triggered on demand from the Actions tab or `gh workflow run`,
instead of pushing a commit just to exercise the pipeline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -5,6 +5,9 @@ on:
     branches: ["main"]
   pull_request:
     branches: ["main"]
+  # Manual trigger: run CI on demand from the Actions tab (or `gh workflow run
+  # ci.yml --ref <branch>`) without pushing a commit just to see it go green.
+  workflow_dispatch:
 
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   contents: read