feat: add system event detection for OpenClaw subagent and gateway messages (#8)

Add detection patterns for OpenClaw-specific system messages so they
render as subtle inline notifications instead of full chat bubbles:

- [System Message] — subagent completion notifications
- [Queued announce messages — batched system messages
- System: [timestamp] — gateway events (e.g. WhatsApp connected)
- Pre-compaction memory flush — internal memory prompts

Includes 6 test cases (5 positive matches + 1 negative for mid-sentence
false positive prevention).

Co-authored-by: togotago <drewmfleury@gmail.com>

src/lib/__tests__/systemEvent.test.ts

@@ -57,6 +57,18 @@ describe('isSystemEvent', () => {
     expect(isSystemEvent('  [EVENT] test')).toBe(true);
     expect(isSystemEvent('\n[cron:x] task')).toBe(true);
   });
+
+  it('detects [System Message] markers (subagent completion notifications)', () => {
+    expect(isSystemEvent('[System Message] Subagent spark completed: task done')).toBe(true);
+    expect(isSystemEvent('[system message] something happened')).toBe(true);
+    expect(isSystemEvent('[SYSTEM MESSAGE] All caps')).toBe(true);
+    expect(isSystemEvent('  [System Message] with leading whitespace')).toBe(true);
+    expect(isSystemEvent('\t[System Message] tab-prefixed')).toBe(true);
+  });
+
+  it('does not falsely detect [System Message] mid-sentence', () => {
+    expect(isSystemEvent('Hello [System Message] this is not a system event')).toBe(false);
+  });
 });
 
 describe('stripWebhookScaffolding', () => {

src/lib/systemEvent.ts

@@ -20,6 +20,18 @@ const SYSTEM_PATTERNS: RegExp[] = [
 
   // System event envelope: [source:xxx]
   /^\[source:\s*\w+\]/i,
+
+  // OpenClaw subagent completion notifications
+  /^\[System Message\]/i,
+
+  // Queued announce messages (batched system messages)
+  /^\[Queued announce messages/i,
+
+  // Gateway system notifications (e.g. "System: [2026-02-18 ...] WhatsApp gateway connected.")
+  /^System:\s*\[\d{4}-\d{2}-\d{2}/,
+
+  // Pre-compaction memory flush prompts
+  /^Pre-compaction memory flush/i,
 ];
 
 export function isSystemEvent(text: string): boolean {