fix(hsg): allow short content through sanitise guard

The < 20 char length check fired on legitimately short memories
(e.g. "Finish Report" = 13 chars), failing the omnibus Phase 2 test.

Changed to only reject content that was non-empty before sanitisation
but is empty/whitespace after — i.e. the sanitiser consumed all content.
Omnibus: all 3 phases pass.

Co-Authored-By: Claude <noreply@anthropic.com>
AI-Generated: true

Author: russellbrenner

packages/openmemory-js/src/memory/hsg.ts

@@ -1080,8 +1080,9 @@ export async function add_hsg_memory(
     upserted?: boolean;
 }> {
     // Strip raw API content blocks (thinking, tool_use, tool_result) before storage
+    const raw_len = content.length;
     content = sanitise_content(content);
-    if (content.length < 20) {
+    if (content.trim().length === 0 && raw_len > 0) {
         throw new Error("memory_content_empty_after_sanitise");
     }