refactor(memory): implement granular cache invalidation for agents

Replaces aggressive `cache.clear()` with per-agent invalidation in `src/services/memory/index.js` to improve performance.
- Adds `prune(predicate)` to `createMemoryCache`.
- Updates `ingestEvents`, `runPruneCycle`, `pinMemory`, `unpinMemory`, and `markMemoryMerged` to invalidate only the affected agent's cache entries.
- Adds unit test for `prune` in `test/memory-cache.test.mjs`.

Co-authored-by: PR0M3TH3AN <[email protected]>

test/memory-cache.test.mjs

@@ -77,3 +77,16 @@ test('blocks decrypted/session-sensitive data from durable promotion without san
   assert.equal(events[1].durable_eligible, true);
   assert.equal(cache.getMetrics().runtime_sensitive_events_blocked, 1);
 });
+
+test('prunes cache entries based on predicate', () => {
+  const cache = createMemoryCache();
+  cache.set('key1', 'value1');
+  cache.set('key2', 'value2');
+  cache.set('other', 'value3');
+
+  cache.prune((key) => key.startsWith('key'));
+
+  assert.equal(cache.get('key1'), undefined);
+  assert.equal(cache.get('key2'), undefined);
+  assert.equal(cache.get('other'), 'value3');
+});