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/gstack-memory-helpers.test.ts

@@ -462,10 +462,13 @@ describe("detectEngineTier", () => {
     // Regression test for #1415: gbrain >=0.25 doctor output dropped the
     // top-level `engine` field. The detect path must fall back to config.json.
     // We force the doctor call to fail (PATH stripped of gbrain) and write a
-    // synthetic config to GBRAIN_HOME so the fallback path is deterministic.
+    // synthetic config under GBRAIN_HOME so the fallback path is
+    // deterministic. Per gbrain's configDir() contract (#2521), GBRAIN_HOME
+    // is a parent dir — the config lives at $GBRAIN_HOME/.gbrain/config.json.
     process.env.PATH = "/nonexistent-no-gbrain-here";
+    mkdirSync(join(testGbrainHome, ".gbrain"), { recursive: true });
     writeFileSync(
-      join(testGbrainHome, "config.json"),
+      join(testGbrainHome, ".gbrain", "config.json"),
       JSON.stringify({ engine: "postgres", database_url: "postgresql://test/example" }),
       "utf-8"
     );
@@ -500,8 +503,9 @@ exit 0
       { mode: 0o755 }
     );
     process.env.PATH = `${binDir}:${process.env.PATH || ""}`;
+    mkdirSync(join(testGbrainHome, ".gbrain"), { recursive: true });
     writeFileSync(
-      join(testGbrainHome, "config.json"),
+      join(testGbrainHome, ".gbrain", "config.json"),
       JSON.stringify({ engine: "pglite" }),
       "utf-8"
     );