fix(gbrain): spawn gbrain + brain-sync through a shell on Windows (#1731)

On Windows, bun/npm install gbrain as a gbrain.cmd/.ps1 shim and gstack-brain-sync
is a bash shebang script. spawnSync/spawn/execFileSync resolve neither without a
shell, so the child spawn failed ENOENT — on the sync orchestrator this surfaced
as 'brain-sync exited undefined' (#1731).

Add NEEDS_SHELL_ON_WINDOWS (process.platform === 'win32') in gbrain-exec and pass
it as shell: to every gbrain/brain-sync child spawn: spawnGbrain, spawnGbrainAsync,
execGbrainText (gbrain-exec), the two sources-list/remove/add spawns (gbrain-sources),
the version + probe spawns (gbrain-local-status), and the two brain-sync spawns in
the orchestrator. POSIX keeps the cheaper no-shell path.

macOS/Linux CI can't exercise the Windows path, so test/gbrain-spawn-windows-shell.ts
is a static-grep tripwire: it fails CI if a gbrain/brain-sync spawn is added without
the shell flag.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/gbrain-exec.ts

@@ -137,6 +137,18 @@ export function buildGbrainEnv(opts: BuildGbrainEnvOptions = {}): NodeJS.Process
   return out;
 }
 
+/**
+ * Windows can't directly spawn the `gbrain` launcher (bun/npm install it as a
+ * `gbrain.cmd`/`.ps1` shim) or a shebang script like the bash `gstack-brain-sync`
+ * — `spawnSync`/`spawn` resolve those only through a shell's PATHEXT + interpreter
+ * lookup. Without `shell: true` the child spawn fails ENOENT, which on the sync
+ * orchestrator surfaced as "brain-sync exited undefined" (#1731). Gate on platform
+ * so POSIX keeps the cheaper no-shell path. Exported so the static-grep tripwire
+ * (test/gbrain-spawn-windows-shell.test.ts) can assert every gbrain/brain-sync
+ * spawn carries it.
+ */
+export const NEEDS_SHELL_ON_WINDOWS = process.platform === "win32";
+
 export interface SpawnGbrainOptions {
   /** Timeout in milliseconds. Defaults to 30s. */
   timeout?: number;
@@ -166,6 +178,7 @@ export function spawnGbrain(args: string[], opts: SpawnGbrainOptions = {}): Spaw
     cwd: opts.cwd,
     stdio: opts.stdio || ["ignore", "pipe", "pipe"],
     env: buildGbrainEnv({ baseEnv: opts.baseEnv, announce: opts.announce }),
+    shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
   });
 }
 
@@ -198,6 +211,7 @@ export function spawnGbrainAsync(
     stdio: opts.stdio || ["ignore", "pipe", "pipe"],
     cwd: opts.cwd,
     env: buildGbrainEnv({ baseEnv: opts.baseEnv, announce: false }),
+    shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
   });
 }
 
@@ -212,5 +226,6 @@ export function execGbrainText(args: string[], opts: SpawnGbrainOptions = {}): s
     cwd: opts.cwd,
     stdio: opts.stdio || ["ignore", "pipe", "pipe"],
     env: buildGbrainEnv({ baseEnv: opts.baseEnv, announce: opts.announce }),
+    shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
   });
 }

lib/gbrain-local-status.ts

@@ -35,7 +35,7 @@ import {
 } from "fs";
 import { homedir } from "os";
 import { dirname, join } from "path";
-import { buildGbrainEnv } from "./gbrain-exec";
+import { buildGbrainEnv, NEEDS_SHELL_ON_WINDOWS } from "./gbrain-exec";
 
 export type LocalEngineStatus =
   | "ok"
@@ -113,6 +113,7 @@ export function resolveGbrainBin(env?: NodeJS.ProcessEnv): string | null {
       timeout: 2_000,
       stdio: ["ignore", "ignore", "ignore"],
       env: e,
+      shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
     });
     result = "gbrain";
   } catch {
@@ -135,6 +136,7 @@ export function readGbrainVersion(env?: NodeJS.ProcessEnv): string {
       timeout: 2_000,
       stdio: ["ignore", "pipe", "ignore"],
       env: e,
+      shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
     });
     result = out.trim().split("\n")[0] || "";
   } catch {
@@ -241,6 +243,7 @@ function freshClassify(env?: NodeJS.ProcessEnv): LocalEngineStatus {
       timeout: PROBE_TIMEOUT_MS,
       stdio: ["ignore", "pipe", "pipe"],
       env: buildGbrainEnv({ baseEnv: env ?? process.env }),
+      shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
     });
     return "ok";
   } catch (err) {

lib/gbrain-sources.ts

@@ -11,6 +11,7 @@
 
 import { execFileSync, spawnSync } from "child_process";
 import { withErrorContext } from "./gstack-memory-helpers";
+import { NEEDS_SHELL_ON_WINDOWS } from "./gbrain-exec";
 
 export interface SourceState {
   /** "absent" — id not registered. "match" — id at expected path. "drift" — id at different path. */
@@ -87,6 +88,7 @@ export function probeSource(id: string, env?: NodeJS.ProcessEnv): SourceState {
       timeout: 30_000,
       stdio: ["ignore", "pipe", "pipe"],
       env,
+      shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
     });
   } catch (err) {
     const e = err as NodeJS.ErrnoException & { stderr?: Buffer };
@@ -160,6 +162,7 @@ export async function ensureSourceRegistered(
         encoding: "utf-8",
         timeout: 30_000,
         env,
+        shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
       });
       if (rm.status !== 0) {
         throw new Error(`gbrain sources remove ${id} failed: ${rm.stderr || rm.stdout || `exit ${rm.status}`}`);
@@ -173,6 +176,7 @@ export async function ensureSourceRegistered(
       encoding: "utf-8",
       timeout: 30_000,
       env,
+      shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
     });
     if (add.status !== 0) {
       throw new Error(`gbrain sources add ${id} failed: ${add.stderr || add.stdout || `exit ${add.status}`}`);
@@ -198,6 +202,7 @@ export function sourcePageCount(id: string, env?: NodeJS.ProcessEnv): number | n
       timeout: 30_000,
       stdio: ["ignore", "pipe", "pipe"],
       env,
+      shell: NEEDS_SHELL_ON_WINDOWS, // #1731: gbrain is a .cmd shim on Windows
     });
   } catch {
     return null;