feat(observer): emit subagent_invoked events from Agent tool_use

Closes brain-retro 2026-05-20 #12 — each Agent tool_use produces a
subagent_invoked event with subagent_type / model (if explicit) /
first 80 chars of description. Visibility from parent Claude's
perspective; full subagent trace lives in subagents/ directory and is
out of scope for this parser.

6 new vitest tests, 315/315 GREEN.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/observer-transcript-parser.test.mjs

@@ -1375,3 +1375,61 @@ describe('parseTranscript — reasoning-tag merges with heuristic (Task 11)', ()
     expect(ep.primary_rationale.boundaries_applied).toContain('Pravila §16');
   });
 });
+
+import { extractAgentInvocations } from './observer-transcript-parser.mjs';
+
+describe('extractAgentInvocations (Task 12)', () => {
+  it('emits subagent_invoked event from Agent tool_use', () => {
+    const turn = [{ message: { role: 'assistant', content: [
+      { type: 'tool_use', name: 'Agent',
+        input: { subagent_type: 'general-purpose', model: 'sonnet', description: 'check files' } }
+    ] } }];
+    const ev = extractAgentInvocations(turn);
+    expect(ev).toHaveLength(1);
+    expect(ev[0].kind).toBe('subagent_invoked');
+    expect(ev[0].subagent_type).toBe('general-purpose');
+    expect(ev[0].model).toBe('sonnet');
+    expect(ev[0].description).toBe('check files');
+  });
+  it('uses "unknown" when subagent_type missing', () => {
+    const turn = [{ message: { role: 'assistant', content: [
+      { type: 'tool_use', name: 'Agent', input: {} }
+    ] } }];
+    expect(extractAgentInvocations(turn)[0].subagent_type).toBe('unknown');
+  });
+  it('truncates description at 80 chars', () => {
+    const turn = [{ message: { role: 'assistant', content: [
+      { type: 'tool_use', name: 'Agent',
+        input: { subagent_type: 'g', description: 'a'.repeat(200) } }
+    ] } }];
+    expect(extractAgentInvocations(turn)[0].description.length).toBe(80);
+  });
+  it('returns empty for non-Agent tool_use', () => {
+    const turn = [{ message: { role: 'assistant', content: [
+      { type: 'tool_use', name: 'Read', input: { file_path: '/a' } }
+    ] } }];
+    expect(extractAgentInvocations(turn)).toEqual([]);
+  });
+  it('safe on null/empty', () => {
+    expect(extractAgentInvocations(null)).toEqual([]);
+    expect(extractAgentInvocations([])).toEqual([]);
+  });
+});
+
+describe('parseTranscript — subagent_invoked events (Task 12)', () => {
+  it('emits subagent_invoked from Agent tool_use', () => {
+    const transcript = [
+      JSON.stringify({ sessionId: 's' }),
+      JSON.stringify({ type: 'user', message: { role: 'user', content: 'делай' }, uuid: 'u1', timestamp: '2026-05-20T00:00:00Z' }),
+      JSON.stringify({ type: 'assistant', message: { role: 'assistant', content: [
+        { type: 'tool_use', id: 't1', name: 'Agent',
+          input: { subagent_type: 'tester', model: 'haiku', description: 'verify spec' } }
+      ] }, uuid: 'u2', timestamp: '2026-05-20T00:00:01Z' }),
+    ].join('\n');
+    const ep = parseTranscript(transcript);
+    const subs = ep.events.filter((e) => e.kind === 'subagent_invoked');
+    expect(subs).toHaveLength(1);
+    expect(subs[0].subagent_type).toBe('tester');
+    expect(subs[0].model).toBe('haiku');
+  });
+});