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.mjs

@@ -532,6 +532,33 @@ export function parseRoutingTag(turn) {
   return null;
 }
 
+/**
+ * Per-Agent-tool_use event (Task 12) — surfaces subagent dispatches in the
+ * episode `events[]`. Captures subagent_type / model (if explicit in input)
+ * / first 80 chars of description.
+ *
+ * Not the full subagent trace (that lives in ~/.claude/projects/.../subagents/);
+ * just visibility from the parent Claude's perspective.
+ */
+export function extractAgentInvocations(turn) {
+  const out = [];
+  for (const e of turn || []) {
+    const content = e && e.message && Array.isArray(e.message.content) ? e.message.content : [];
+    for (const b of content) {
+      if (b && b.type === 'tool_use' && b.name === 'Agent') {
+        const inp = b.input || {};
+        out.push({
+          kind: 'subagent_invoked',
+          subagent_type: inp.subagent_type || 'unknown',
+          model: inp.model || null,
+          description: typeof inp.description === 'string' ? inp.description.slice(0, 80) : '',
+        });
+      }
+    }
+  }
+  return out;
+}
+
 const REASONING_TAG_RE =
   /<!--\s*reasoning:\s*triggers="([^"]*)"\s+candidates="([^"]*)"\s+boundaries="([^"]*)"\s*-->/;
 
@@ -611,6 +638,7 @@ export function parseTranscript(transcriptText, fallbackSessionId = null) {
   }
   events.push(...extractProcessEvents(turn, broken, total, durationMs));
   events.push(...extractAskUserQuestionEvents(turn));
+  events.push(...extractAgentInvocations(turn));
 
   const usedSuperpowers = skills.some((s) => String(s).startsWith(SUPERPOWERS_PREFIX));
   const prompt = promptText(entries[start]);