security: pass cwd to git via execFileSync, not interpolation through /bin/sh

`bin/gstack-memory-ingest.ts:632-643` ran `execSync(\`git -C ${JSON.stringify(cwd)}
remote get-url origin 2>/dev/null\`, ...)`. JSON.stringify escapes `"` and `\`
but not `$` or backticks, so a `cwd` of `"$(touch /tmp/marker)"` survived JSON
quoting and detonated under /bin/sh's command-substitution-inside-double-quotes.

`cwd` originates from transcript JSONL records under
`~/.claude/projects/<encoded-cwd>/<uuid>.jsonl` and
`~/.codex/sessions/YYYY/MM/DD/rollout-*.jsonl`. The walker grabs the first
`.cwd` it sees per session. That's an untrusted surface in the gstack threat
model — the L1-L6 sidebar security stack exists exactly because agent
transcripts can carry attacker-influenced text. Two pivots above the local
same-uid bar: (a) prompt-injection appending `cwd="$(...)"` to the active
session log turns the next /sync-gbrain run into RCE under the user's uid;
(b) cross-machine transcript share (a colleague's `.claude/projects` snippet
untar'd into HOME, a documented gbrain dogfooding shape) → RCE on first sync.

Fix swaps the one execSync for `execFileSync("git", ["-C", cwd, "remote",
"get-url", "origin"], ...)`. No shell, argv passed directly to git. The same
module already uses execFileSync for `gbrainAvailable()` (line 762 pre-patch)
and `gbrainPutPage()` (line 816 pre-patch) — this single execSync was the
outlier.

Test: `gstack-memory-ingest security: untrusted cwd cannot trigger shell
substitution` plants a Claude-Code-shaped JSONL with cwd=`$(touch <marker>)`
and asserts the marker file is not created after `--incremental --quiet`.
Negative control: with the patch reverted, the test fails (marker created);
with the patch applied, it passes (18/18 in test/gstack-memory-ingest.test.ts).

bin/gstack-memory-ingest.ts

@@ -632,9 +632,16 @@ function extractContentText(rec: any): string {
 function resolveGitRemote(cwd: string): string {
   if (!cwd) return "";
   try {
-    const out = execSync(`git -C ${JSON.stringify(cwd)} remote get-url origin 2>/dev/null`, {
+    // execFileSync (no shell) so `cwd` cannot trigger command substitution.
+    // Transcript JSONL records are an untrusted surface (a poisoned `.cwd`
+    // value containing `"$(...)"` survived `JSON.stringify` interpolation
+    // into a `/bin/sh -c` context, since JSON quoting does not escape `$`
+    // or backticks). Mirrors the execFileSync pattern this module already
+    // uses for `gbrainAvailable()` (line 762) and `gbrainPutPage()` (line 816).
+    const out = execFileSync("git", ["-C", cwd, "remote", "get-url", "origin"], {
       encoding: "utf-8",
       timeout: 2000,
+      stdio: ["ignore", "pipe", "ignore"],
     });
     return canonicalizeRemote(out.trim());
   } catch {