fix: resolve GBRAIN_HOME with gbrain's parent-dir semantics (#2521)

gstack treated GBRAIN_HOME as the config directory; gbrain's configDir()
treats it as the PARENT and always appends `.gbrain` itself (the contract
is explicit in gbrain's source: GBRAIN_HOME=/tmp/x → /tmp/x/.gbrain/
config.json). With GBRAIN_HOME set, gstack classified engine status from
a file gbrain never reads — the probe's two halves (file checks vs the
spawned `gbrain sources list`) looked at DIFFERENT installs, so any
resulting status was arbitrary: missing-config/broken-config against
healthy installs, or a thin-client marker gstack saw that gbrain itself
reported as "No brain configured".

New shared resolver `gbrainConfigDir()` in lib/gbrain-exec.ts is the
single source of truth. All seven gstack sites route through the contract:

- lib/gbrain-local-status.ts gbrainConfigPath (the classifier's file half)
- bin/gstack-gbrain-detect GBRAIN_CONFIG + readRemoteMcpUrl
- lib/gbrain-exec.ts buildGbrainEnv (the probe's DATABASE_URL seed —
  fixing only the classifier would have left the split-brain in the
  spawn half, flagged by the reporter)
- lib/gbrain-guards.ts gbrainHome (clones-dir + autopilot-lock paths)
- lib/gstack-memory-helpers.ts gbrainConfigPath (engine-tier fallback)
- bin/gstack-gbrain-install pre-doctor config check (shell)

Unit tests cover GBRAIN_HOME set (config found at $GBRAIN_HOME/.gbrain),
the old flat layout explicitly NOT read (both classifier and
buildGbrainEnv), and unset (~/.gbrain unchanged). Existing fixtures that
encoded the deviant flat layout are updated to gbrain's contract.

Root-cause analysis by @d-danielsun in #2521.

Deviation from the 3-site plan spec: the same deviant resolution existed
in four more sites (buildGbrainEnv, gbrain-guards, memory-helpers,
gbrain-install); fixing only three would have left gstack disagreeing
with itself as well as with gbrain, so the whole class moved to the
shared resolver in one change.

Fixes #2521

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/build-gbrain-env.test.ts

@@ -77,17 +77,34 @@ describe("buildGbrainEnv", () => {
     expect(result.DATABASE_URL).toBe("postgresql://app/db");
   });
 
-  it("honors GBRAIN_HOME when set (config aligned with detectEngineTier)", () => {
-    // Move the config to an alternate dir; set GBRAIN_HOME to point at it.
+  it("honors GBRAIN_HOME when set, with gbrain's parent-dir semantics (#2521)", () => {
+    // Move the config to an alternate dir; set GBRAIN_HOME to point at its
+    // PARENT — gbrain's configDir() appends `.gbrain` itself, so
+    // GBRAIN_HOME=/x reads /x/.gbrain/config.json.
     const altGbrainHome = join(home, "alt-gbrain");
-    mkdirSync(altGbrainHome, { recursive: true });
-    writeFileSync(join(altGbrainHome, "config.json"), JSON.stringify({ database_url: "postgresql://alt/db" }));
+    mkdirSync(join(altGbrainHome, ".gbrain"), { recursive: true });
+    writeFileSync(
+      join(altGbrainHome, ".gbrain", "config.json"),
+      JSON.stringify({ database_url: "postgresql://alt/db" }),
+    );
     // No file at the default ~/.gbrain location.
     const baseEnv = { HOME: home, GBRAIN_HOME: altGbrainHome };
     const result = buildGbrainEnv({ baseEnv });
     expect(result.DATABASE_URL).toBe("postgresql://alt/db");
   });
 
+  it("ignores a config at $GBRAIN_HOME/config.json — gbrain never reads that file (#2521)", () => {
+    const altGbrainHome = join(home, "alt-gbrain-flat");
+    mkdirSync(altGbrainHome, { recursive: true });
+    writeFileSync(
+      join(altGbrainHome, "config.json"),
+      JSON.stringify({ database_url: "postgresql://alt/db" }),
+    );
+    const baseEnv = { HOME: home, GBRAIN_HOME: altGbrainHome };
+    const result = buildGbrainEnv({ baseEnv });
+    expect(result.DATABASE_URL).toBeUndefined();
+  });
+
   it("returns a fresh env object — never the caller's env by identity", () => {
     // Codex review #11: object-identity equality lets later mutation of the
     // returned env leak back into the caller's view. The helper MUST clone.