fix(brain-context-load): probe gbrain via execFile, not shell builtin (#1559)

gbrainAvailable() used `execFileSync("command", ["-v", "gbrain"])`,
which fails in any environment where the `command` builtin isn't on
the spawned process's PATH (most non-interactive shells). The probe
then reported gbrain as missing even when it was installed, and
context-load silently skipped vector/list queries.

Fix: probe `gbrain --version` directly with a 500ms timeout (matching
the rest of the file's MCP_TIMEOUT_MS). Same semantics, works
everywhere execFile works.

Contributed by @jbetala7 via #1560. Closes #1559.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

bin/gstack-brain-context-load.ts

@@ -192,7 +192,10 @@ function resolveSkillFile(args: CliArgs): string | null {
 
 function gbrainAvailable(): boolean {
   try {
-    execFileSync("command", ["-v", "gbrain"], { stdio: "ignore" });
+    execFileSync("gbrain", ["--version"], {
+      stdio: "ignore",
+      timeout: MCP_TIMEOUT_MS,
+    });
     return true;
   } catch {
     return false;