feat(ci): OTEL timing receiver for check-dagger

Adds ci/otelrecv/main.go — a minimal OTLP HTTP/JSON trace receiver that
listens on a random port (port 0) so parallel runs never collide.

The check-dagger Taskfile task now starts the receiver in the background,
passes the port via a mktemp file, runs dagger with OTEL env vars set,
then prints a per-span timing report on shutdown. Falls back to plain
dagger call when Go is not available (e.g. CI containers without Go).

First run will show raw attribute keys so we can learn Dagger's exact
telemetry format and refine the cached/live detection logic.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Taskfile.yml

@@ -252,9 +252,32 @@ tasks:
       - dagger call --progress=plain -q -m ci --source=. publish-website --ssh-key env:SSH_PRIVATE_KEY --ssh-user "$SSH_USER" --ssh-host "$SSH_HOST"
 
   check-dagger:
-    desc: Run full check suite via Dagger
+    desc: Run full check suite via Dagger (with OTEL timing report if Go is available)
     cmds:
-      - dagger call --progress=plain -q -m ci --source=. check
+      - |
+        if ! command -v go >/dev/null 2>&1; then
+          dagger call --progress=plain -q -m ci --source=. check
+          exit $?
+        fi
+        PORTFILE=$(mktemp)
+        TIMINGFILE=$(mktemp)
+        (cd ci && go run ./otelrecv/ --port-file="$PORTFILE") > "$TIMINGFILE" &
+        RECV_PID=$!
+        cleanup() {
+          kill "$RECV_PID" 2>/dev/null
+          wait "$RECV_PID" 2>/dev/null
+          echo ""
+          cat "$TIMINGFILE"
+          rm -f "$PORTFILE" "$TIMINGFILE"
+        }
+        trap cleanup EXIT
+        until [ -s "$PORTFILE" ]; do sleep 0.05; done
+        PORT=$(cat "$PORTFILE")
+        RC=0
+        OTEL_EXPORTER_OTLP_ENDPOINT="http://127.0.0.1:$PORT" \
+        OTEL_EXPORTER_OTLP_PROTOCOL="http/json" \
+        dagger call --progress=plain -q -m ci --source=. check || RC=$?
+        exit $RC
 
   integration-android:
     desc: UI integration tests on a connected Android emulator (Stalwart on host, emulator reaches it via 10.0.2.2)