fix(sync): run gstack-brain-sync through bash, not cmd.exe, on Windows

The brain-sync stage failed on EVERY Windows run with "is not
recognized as an internal or external command", so /sync-gbrain always
reported ERR brain-sync among otherwise green stages.

#1731 gave these spawns shell: NEEDS_SHELL_ON_WINDOWS. That is correct
for the gbrain.cmd shim and does nothing here: shell:true routes through
cmd.exe, which resolves .cmd/.bat via PATHEXT but has no concept of a
shebang, so an extension-less bash script is rejected outright. A .cmd
shim needs a shell; a shebang script needs an interpreter. The two cases
look identical and are not.

The failure was quiet rather than loud. artifacts_sync_mode defaults to
pushing curated artifacts to git, so a Windows user's learnings piled up
uncommitted in ~/.gstack indefinitely while the sync report showed one
red line out of four.

New bashScriptInvocation() resolves Git for Windows' bash explicitly and
passes the script as argv[0]. It prefers Git bash over a bare `bash` on
PATH because WindowsApps ships a bash.exe that is the WSL launcher, which
would read C:\... as a Linux path; GSTACK_BASH overrides for unusual
installs; forward slashes because bash treats backslashes as escapes; and
it returns null when no bash exists so the stage says so plainly instead
of surfacing an unactionable spawn error.

The #1731 tripwire asserted the shape that does not work, so it now
asserts the opposite (never a raw spawnSync(brainSyncPath, ...)) and six
unit tests cover the resolver.

Verified on Windows: the stage now reports "OK brain-sync curated
artifacts pushed (4.2s)" and the artifacts repo committed + pushed on its
own. Affected-test set unchanged at 14 pre-existing failures before and
after, with 6 new passing tests.

bin/gstack-gbrain-sync.ts

@@ -41,7 +41,7 @@ import { ensureSourceRegistered, sourcePageCount, parseSourcesList, cycleComplet
 import { detectAutopilot, decideSourceRemove, decideCodeSync } from "../lib/gbrain-guards";
 import { writeReceipt } from "../lib/egress-receipt";
 import { localEngineStatus, type LocalEngineStatus } from "../lib/gbrain-local-status";
-import { buildGbrainEnv, spawnGbrain, execGbrainJson, NEEDS_SHELL_ON_WINDOWS } from "../lib/gbrain-exec";
+import { buildGbrainEnv, spawnGbrain, execGbrainJson, NEEDS_SHELL_ON_WINDOWS, bashScriptInvocation } from "../lib/gbrain-exec";
 import { repoPolicyTier as sharedRepoPolicyTier } from "../lib/gbrain-repo-policy-client";
 import { checkOwnedStagingDir } from "../lib/staging-guard";
 
@@ -1245,18 +1245,31 @@ function runBrainSyncPush(args: CliArgs): StageResult {
     return { name: "brain-sync", ran: false, ok: true, duration_ms: 0, summary: "skipped (gstack-brain-sync not installed)" };
   }
 
-  // #1731: gstack-brain-sync is a bash shebang script; Windows can't spawn it
-  // without a shell, which surfaced as "brain-sync exited undefined".
-  spawnSync(brainSyncPath, ["--discover-new"], {
-    stdio: args.quiet ? ["ignore", "ignore", "ignore"] : ["ignore", "inherit", "inherit"],
-    timeout: 60 * 1000,
-    shell: NEEDS_SHELL_ON_WINDOWS,
-  });
-  const result = spawnSync(brainSyncPath, ["--once"], {
-    stdio: args.quiet ? ["ignore", "ignore", "ignore"] : ["ignore", "inherit", "inherit"],
-    timeout: 60 * 1000,
-    shell: NEEDS_SHELL_ON_WINDOWS,
-  });
+  // gstack-brain-sync is a bash shebang script, so it needs an INTERPRETER, not
+  // a shell. #1731 gave it `shell: NEEDS_SHELL_ON_WINDOWS`, which is right for
+  // the gbrain.cmd shim and useless here: cmd.exe resolves .cmd/.bat via PATHEXT
+  // and rejects an extension-less shebang script outright ("is not recognized as
+  // an internal or external command"), so this stage failed on EVERY Windows run
+  // while looking like a single red line in an otherwise green report. See
+  // bashScriptInvocation.
+  const discover = bashScriptInvocation(brainSyncPath, ["--discover-new"]);
+  const once = bashScriptInvocation(brainSyncPath, ["--once"]);
+  if (!discover || !once) {
+    return {
+      name: "brain-sync",
+      ran: false,
+      ok: true,
+      duration_ms: Date.now() - t0,
+      summary: "skipped (no bash found; set GSTACK_BASH to your Git bash.exe)",
+    };
+  }
+
+  const stdio: "ignore"[] | ("ignore" | "inherit")[] = args.quiet
+    ? ["ignore", "ignore", "ignore"]
+    : ["ignore", "inherit", "inherit"];
+
+  spawnSync(discover.cmd, discover.argv, { stdio, timeout: 60 * 1000, shell: discover.shell });
+  const result = spawnSync(once.cmd, once.argv, { stdio, timeout: 60 * 1000, shell: once.shell });
 
   return {
     name: "brain-sync",