ci: don't cancel-in-progress for main-branch runs (only pull_request)

Two back-to-back merges to main cancelled the first merge's deploy: the second
push started a new run in the same concurrency group with cancel-in-progress,
killing the first's Deploy Agent mid-build and leaving the agent un-deployed.

Make cancellation conditional on the event being a pull_request, so stale PR CI
still cancels but main-branch deploys always run to completion.

Refs #118.

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

.gitea/workflows/ci.yml

@@ -29,10 +29,13 @@ env:
   CARGO_NET_RETRY: "10"
   CARGO_HTTP_MULTIPLEXING: "false"
 
-# Cancel in-progress runs for the same branch/PR
+# Cancel superseded PR runs, but NEVER cancel main-branch runs — those build and
+# deploy per-service images, and cancelling one merge's deploy when the next
+# merge lands leaves a service un-deployed (as happened between two back-to-back
+# merges). So cancel-in-progress only for pull_request events.
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
   # ---------------------------------------------------------------------------