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>

lib/gstack-memory-helpers.ts

@@ -19,6 +19,7 @@
 
 import { existsSync, readFileSync, writeFileSync, mkdirSync, statSync } from "fs";
 import { appendJsonl } from "./jsonl-store";
+import { gbrainConfigDir } from "./gbrain-exec";
 import { dirname, join } from "path";
 import { execFileSync } from "child_process";
 import { homedir } from "os";
@@ -256,12 +257,13 @@ export function detectEngineTier(): EngineDetect {
 }
 
 // Returns gbrain's config.json path, honoring GBRAIN_HOME env var with a
-// fallback to ~/.gbrain. gbrain >=0.25 dropped the top-level `engine` field
+// fallback to ~/.gbrain. Resolution matches gbrain's own configDir()
+// contract (#2521): GBRAIN_HOME is a parent dir, `.gbrain` is appended.
+// gbrain >=0.25 dropped the top-level `engine` field
 // from doctor output, so this file is the only reliable source for engine
 // detection on that version. See #1415.
 function gbrainConfigPath(): string {
-  const root = process.env.GBRAIN_HOME || join(homedir(), ".gbrain");
-  return join(root, "config.json");
+  return join(gbrainConfigDir(process.env), "config.json");
 }
 
 // Best-effort JSONL append to ~/.gstack/.gbrain-errors.jsonl. Never throws.