fix: use null-byte delimiter in observation content hash to prevent collisions

Fields concatenated without separators allowed different tuples to produce
identical hashes (e.g. session="ab", title="cd" vs session="abc", title="d").
This could cause legitimate observations to be silently deduplicated.

Join with \x00 so field boundaries are unambiguous.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/sqlite/observations/store.ts

@@ -22,7 +22,7 @@ export function computeObservationContentHash(
   narrative: string | null
 ): string {
   return createHash('sha256')
-    .update((memorySessionId || '') + (title || '') + (narrative || ''))
+    .update([memorySessionId || '', title || '', narrative || ''].join('\x00'))
     .digest('hex')
     .slice(0, 16);
 }