fix(gbrain-sync): centralize gbrain spawn surface + seed DATABASE_URL

Cherry-picked from #1508 by jasshultz, restructured per codex review #4
and #7 to widen scope and centralize the spawn surface.

The bug: gbrain auto-loads .env.local from cwd via dotenv. When
/sync-gbrain runs inside a Next.js / Prisma / Rails project whose
.env.local defines its own DATABASE_URL (pointing at the app's local
DB), gbrain reads that value instead of its own
~/.gbrain/config.json — auth fails, code + memory stages crash.

This commit:

- Adds lib/gbrain-exec.ts: buildGbrainEnv, spawnGbrain, execGbrainJson,
  execGbrainText, spawnGbrainAsync (the last one for memory-ingest's
  streaming gbrain import call). buildGbrainEnv seeds DATABASE_URL from
  ${GBRAIN_HOME:-$HOME/.gbrain}/config.json, returns a fresh env object
  (never the caller's by identity — codex review #11), and honors the
  GSTACK_RESPECT_ENV_DATABASE_URL=1 escape hatch.

- Routes every gbrain spawn in bin/gstack-gbrain-sync.ts and
  bin/gstack-memory-ingest.ts through the helpers. Both files now own
  zero direct spawnSync("gbrain"|spawn("gbrain"|execFileSync("gbrain"
  call sites.

- Threads buildGbrainEnv into the spawnSync("bun", [memory-ingest], ...)
  grandchild in runMemoryIngest (codex review #7). Without this, the
  parent fix is half-baked — the bun child inherits a clean env but
  needs DATABASE_URL pre-seeded too. spawnGbrainAsync inside
  memory-ingest provides defense in depth for standalone invocations.

- Adds GBRAIN_HOME support — aligns with detectEngineTier (already
  honors GBRAIN_HOME) so all gstack-side gbrain calls agree on which
  config file matters. Resolves baseEnv.HOME first, then homedir(), so
  test injection works without process-wide HOME mutation.

- Adds test/build-gbrain-env.test.ts: 10 unit tests covering all five
  env-seeding branches (seed from config / override caller /
  GSTACK_RESPECT escape hatch / missing config / unparseable config /
  no database_url field / GBRAIN_HOME path / object-identity guard /
  unrelated-vars preservation / idempotent-when-matches).

- Adds test/gbrain-exec-invariant.test.ts: static-source check that
  greps both bin/gstack-gbrain-sync.ts and bin/gstack-memory-ingest.ts
  for direct spawnSync("gbrain"|spawn("gbrain"|execFileSync("gbrain"|
  execSync(...gbrain matches and fails the build if any are found.
  Refactor-proof against future contributors adding a new gbrain spawn
  without env threading.

The invariant is intentionally narrow — only the two files where the
DATABASE_URL bug actually hurts users are guarded. Migrating the
spawn sites in lib/gbrain-local-status.ts, lib/gstack-memory-helpers.ts,
and bin/gstack-brain-context-load.ts is a follow-up.

Co-Authored-By: Jason Shultz <jasshultz@gmail.com>
Co-Authored-By: Claude <noreply@anthropic.com>

bin/gstack-memory-ingest.ts

@@ -64,6 +64,7 @@ import {
   detectEngineTier,
   withErrorContext,
 } from "../lib/gstack-memory-helpers";
+import { execGbrainText, spawnGbrainAsync } from "../lib/gbrain-exec";
 
 // ── Types ──────────────────────────────────────────────────────────────────
 
@@ -813,11 +814,10 @@ function gbrainAvailable(): boolean {
     // `import <dir>` (batch markdown import via path-authoritative slugs).
     // If absent, we surface a single clean error here rather than failing
     // the whole stage with a confusing usage message from gbrain itself.
-    const help = execFileSync("gbrain", ["--help"], {
-      encoding: "utf-8",
-      timeout: 5000,
-      stdio: ["ignore", "pipe", "pipe"],
-    });
+    // `gbrain --help` probes only CLI availability, not DB connectivity, so
+    // it doesn't strictly need DATABASE_URL. But routing through the helper
+    // keeps the invariant test from chasing exceptions per call site.
+    const help = execGbrainText(["--help"], { timeout: 5000 });
     _gbrainAvailability = /^\s+import\s/m.test(help);
   } catch {
     _gbrainAvailability = false;
@@ -1316,11 +1316,11 @@ function runGbrainImport(
 ): Promise<{ status: number | null; stdout: string; stderr: string }> {
   installSignalForwarder();
   return new Promise((resolve) => {
-    const child = spawn(
-      "gbrain",
-      ["import", stagingDir, "--no-embed", "--json"],
-      { stdio: ["ignore", "pipe", "pipe"] },
-    );
+    // Seed DATABASE_URL from gbrain's own config so this stage works
+    // inside Next.js / Prisma / Rails projects with their own
+    // .env.local (codex review #7 — defense in depth on top of the
+    // parent gstack-gbrain-sync seeding the bun grandchild's env).
+    const child = spawnGbrainAsync(["import", stagingDir, "--no-embed", "--json"]);
     _activeImportChild = child;
     let stdout = "";
     let stderr = "";