feat(coding-agent): env-driven OTLP trace export for headless hosts

When OTEL_EXPORTER_OTLP_ENDPOINT (or _TRACES_ENDPOINT) is set, register a
global OTLP/proto TracerProvider and switch on the agent loop's telemetry
so its invoke_agent / chat / execute_tool spans reach a collector instead
of a no-op tracer. Honors the standard OTEL_* env contract
(OTEL_SERVICE_NAME, OTEL_SDK_DISABLED, OTEL_TRACES_EXPORTER=none) and the
OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT capture toggle; no-op
when no endpoint is configured.

Uses the exporter-trace-otlp-proto 2.x line, which exports cleanly under
Bun (the 1.x line deadlocks). Adds a flushTelemetryExport() helper and a
periodic + signal-driven flush so long-lived processes surface spans.

This makes the existing telemetry usable from hosts that spawn omp as a
child process (e.g. an ACP harness), where an in-process provider
registered by the parent can't reach the child process.

packages/coding-agent/test/otel-export-probe.ts

@@ -0,0 +1,54 @@
+/**
+ * Positive-path probe for the OTLP trace exporter, run as a subprocess by
+ * telemetry-export.test.ts. Keeping it out-of-process means the global
+ * TracerProvider singleton that initTelemetryExport() registers never leaks
+ * into the test runner.
+ *
+ * Stands up a loopback OTLP/proto receiver, points the standard env var at it,
+ * registers the provider, emits a span through the same tracer name the agent
+ * core uses, flushes, and exits 0 only if the receiver got a non-empty
+ * protobuf POST at /v1/traces.
+ */
+import { trace } from "@opentelemetry/api";
+import { flushTelemetryExport, initTelemetryExport, isTelemetryExportEnabled } from "../src/telemetry-export";
+
+let received = false;
+
+const server = Bun.serve({
+	port: 0,
+	async fetch(req) {
+		const path = new URL(req.url).pathname;
+		if (req.method === "POST" && path.endsWith("/v1/traces")) {
+			const body = await req.arrayBuffer();
+			if (body.byteLength > 0 && req.headers.get("content-type") === "application/x-protobuf") {
+				received = true;
+			}
+			return new Response('{"partialSuccess":{}}', {
+				status: 200,
+				headers: { "content-type": "application/json" },
+			});
+		}
+		return new Response("not found", { status: 404 });
+	},
+});
+
+process.env.OTEL_EXPORTER_OTLP_TRACES_ENDPOINT = `http://localhost:${server.port}/v1/traces`;
+process.env.OTEL_SERVICE_NAME = "oh-my-pi-export-probe";
+
+initTelemetryExport();
+if (!isTelemetryExportEnabled()) {
+	console.error("PROBE: provider did not register");
+	await server.stop(true);
+	process.exit(2);
+}
+
+const span = trace.getTracer("@oh-my-pi/pi-agent-core").startSpan("agent.llm_call");
+span.setAttribute("gen_ai.system", "probe");
+span.setAttribute("gen_ai.request.model", "claude-haiku-4-5");
+span.end();
+
+await flushTelemetryExport();
+await server.stop(true);
+
+console.log(received ? "PROBE: RECEIVED" : "PROBE: NO_EXPORT");
+process.exit(received ? 0 : 1);