fix(query): drain cache writes before CLI exit

The query cache write was fired with `void promise.catch(...)` — true
fire-and-forget. On a fast CLI invocation (`gbrain query <q>` exits in
~50ms), the process terminates before the cache write commits. Result:
the cache effectively never warms from CLI use; every query is a miss.

`awaitPendingSearchCacheWrites()` tracks each in-flight cache write in a
module-level Set. The CLI dispatcher awaits the set after `query`
finishes formatting output but before the process exits. MCP server path
unchanged (long-lived process, fire-and-forget remains correct).

Cherry-picked from PR #1125.

Co-Authored-By: hnshah <hnshah@users.noreply.github.com>

src/cli.ts

@@ -169,6 +169,10 @@ async function main() {
     const result = JSON.parse(JSON.stringify(rawResult));
     const output = formatResult(op.name, result);
     if (output) process.stdout.write(output);
+    if (op.name === 'query') {
+      const { awaitPendingSearchCacheWrites } = await import('./core/search/hybrid.ts');
+      await awaitPendingSearchCacheWrites();
+    }
   } catch (e: unknown) {
     if (e instanceof OperationError) {
       console.error(`Error [${e.code}]: ${e.message}`);

src/core/search/hybrid.ts

@@ -31,6 +31,17 @@ import {
 
 const RRF_K = 60;
 const COMPILED_TRUTH_BOOST = 2.0;
+const pendingCacheWrites = new Set<Promise<unknown>>();
+
+export async function awaitPendingSearchCacheWrites(): Promise<void> {
+  if (pendingCacheWrites.size === 0) return;
+  await Promise.allSettled([...pendingCacheWrites]);
+}
+
+function trackCacheWrite(promise: Promise<unknown>): void {
+  pendingCacheWrites.add(promise);
+  promise.finally(() => pendingCacheWrites.delete(promise)).catch(() => { /* swallow */ });
+}
 /**
  * Backlink boost coefficient. Score is multiplied by (1 + BACKLINK_BOOST_COEF * log(1 + count)).
  * - 0 backlinks: factor = 1.0 (no boost).
@@ -861,9 +872,11 @@ export async function hybridSearchCached(
     results.length > 0 &&
     (innerMeta?.vector_enabled ?? false)
   ) {
-    void cache
-      .store(query, queryEmbedding, results, finalMeta, { sourceId: opts?.sourceId, knobsHash: cacheKnobsHash })
-      .catch(() => { /* swallow */ });
+    trackCacheWrite(
+      cache
+        .store(query, queryEmbedding, results, finalMeta, { sourceId: opts?.sourceId, knobsHash: cacheKnobsHash })
+        .catch(() => { /* swallow */ }),
+    );
   }
 
   return budgeted;