fix(gbrain-detect): ignore doctor stderr so a chatty doctor cannot overflow the spawn buffer

stderr over spawnSync's 1 MB maxBuffer made the call fail with ENOBUFS and
dropped the stdout report, turning a healthy brain into a Step 5 STOP.

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

bin/gstack-gbrain-detect

@@ -147,7 +147,10 @@ interface DoctorVerdict {
 function runDoctor(extraArgs: string[], timeoutMs: number): DoctorVerdict {
   let r: ReturnType<typeof spawnGbrain>;
   try {
-    r = spawnGbrain(["doctor", "--json", ...extraArgs, "--scope=brain"], { timeout: timeoutMs });
+    r = spawnGbrain(["doctor", "--json", ...extraArgs, "--scope=brain"], {
+      timeout: timeoutMs,
+      stdio: ["ignore", "pipe", "ignore"],
+    });
   } catch (e) {
     const error = `gbrain doctor could not start: ${e instanceof Error ? e.message : String(e)}`;
     return { brain_ok: false, status: null, brain_failures: [], skill_failures: [], report: null, error };
@@ -158,11 +161,8 @@ function runDoctor(extraArgs: string[], timeoutMs: number): DoctorVerdict {
   } catch {}
   if (!report || typeof report !== "object") {
     const cause = r.error ? r.error.message : r.signal ? `killed by ${r.signal}` : `exit ${r.status}`;
-    const stderr = (r.stderr || "").trim().split("\n").slice(-5).join("\n");
-    return {
-      brain_ok: false, status: null, brain_failures: [], skill_failures: [], report: null,
-      error: `gbrain doctor printed no JSON report (${cause})${stderr ? `: ${stderr}` : ""}`,
-    };
+    const error = `gbrain doctor printed no JSON report (${cause})`;
+    return { brain_ok: false, status: null, brain_failures: [], skill_failures: [], report: null, error };
   }
   const failed = (Array.isArray(report.checks) ? report.checks : []).filter((c) => c?.status === "fail");
   const skill_failures = failed.filter((c) => c.category === "skill").map((c) => c.name ?? "unnamed");