fix(ci): kill stale platform-server before binding port 8080

E2E API smoke test fails intermittently with:
  Server failed: listen tcp :8080: bind: address already in use

Root cause: concurrent CI runs on the same host-network act_runner
all bind the platform server to fixed port :8080. When a previous
run is cancelled before the "Stop platform" step runs, its process
lingers on :8080 and the new run fails to bind.

Fix: add a pre-start step that probes :8080 and kills any stale
platform-server via /proc scan. This is safe (no false positives
— only kills if the port is actually in use) and requires no extra
tools beyond curl+grep+kill which are universally available on
Ubuntu/Debian runners.

Refs: internal#374
Fixes: internal#374

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/e2e-api.yml

@@ -242,6 +242,28 @@ jobs:
         if: needs.detect-changes.outputs.api == 'true'
         working-directory: workspace-server
         run: go build -o platform-server ./cmd/server
+      - name: Free port 8080 before start
+        if: needs.detect-changes.outputs.api == 'true'
+        run: |
+          # Kill any stale platform-server from a previous run that failed to
+          # clean up (e.g. runner was cancelled before the Stop step ran).
+          # Concurrent runs on the same host-network runner all bind :8080.
+          # Try curl first (cheap), kill if port is occupied.
+          if curl -sf http://127.0.0.1:8080/health > /dev/null 2>&1; then
+            echo "Port 8080 in use — killing stale platform-server"
+            # /proc scan — works on any Linux without pkill/lsof/ss.
+            # comm field is truncated to 15 chars: "platform-serve" matches.
+            # shellcheck disable=SC2013
+            for pid in $(grep -l "platform-serve" /proc/[0-9]*/comm 2>/dev/null); do
+              kpid="${pid%/comm}"
+              kpid="${kpid##*/}"
+              echo "Killing stale process $kpid"
+              kill "$kpid" 2>/dev/null || true
+            done
+            sleep 2  # Wait for port to release.
+          else
+            echo "Port 8080 is free"
+          fi
       - name: Start platform (background)
         if: needs.detect-changes.outputs.api == 'true'
         working-directory: workspace-server