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/code-intelligence.test.ts

@@ -953,7 +953,9 @@ exit 1
       ...process.env,
       GSTACK_HOME: home,
       HOME: home,
-      GBRAIN_HOME: gbrainHome,
+      // #2521: GBRAIN_HOME is the PARENT of .gbrain per gbrain's configDir()
+      // contract — pointing it at `home` resolves to home/.gbrain/config.json.
+      GBRAIN_HOME: home,
       PATH: `${shimDir}:${process.env.PATH}`,
       // Dead loopback port → the Sourcebot probe fails fast + deterministically
       // (connection refused) instead of poking whatever operator dev server