feat(observer): differentiate error events by tool + summary

Closes brain-retro 2026-05-20 #7 — each tool_result.is_error now emits
{ kind:'error', tool:<name>, summary:<first 80 chars> }. Allows
aggregation by tool (Bash/Edit/Read) + cause prefix (ENOENT/timeout/
'String to replace not found').

Required updating existing 'emits error events for tool_result with
is_error' test assertion (old shape had bare 'message' field).

4 new vitest tests + 1 existing relaxed, 286/286 GREEN.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/observer-transcript-parser.mjs

@@ -127,7 +127,16 @@ export function classifyTask(text) {
 function collectToolUse(entries) {
   const skills = [];
   const counts = {};
-  let errorCount = 0;
+  const errors = [];
+  const idToTool = {};
+  // First pass — build id→tool name map (tool_results may reference tools across messages)
+  for (const e of entries) {
+    const content = e && e.message && Array.isArray(e.message.content) ? e.message.content : [];
+    for (const b of content) {
+      if (b && b.type === 'tool_use') idToTool[b.id] = b.name || 'unknown';
+    }
+  }
+  // Second pass — accumulate counts + per-error attribution
   for (const e of entries) {
     const content = e && e.message && Array.isArray(e.message.content) ? e.message.content : [];
     for (const block of content) {
@@ -139,11 +148,15 @@ function collectToolUse(entries) {
           skills.push((block.input && block.input.skill) || 'unknown');
         }
       } else if (block.type === 'tool_result' && block.is_error === true) {
-        errorCount += 1;
+        const tool = idToTool[block.tool_use_id] || 'unknown';
+        const c = block.content;
+        const text = typeof c === 'string' ? c
+          : (Array.isArray(c) ? c.map((b) => (b && typeof b.text === 'string') ? b.text : '').join(' ') : '');
+        errors.push({ tool, summary: text.slice(0, 80) });
       }
     }
   }
-  return { skills, counts, errorCount };
+  return { skills, counts, errors };
 }
 
 const FILE_TOOLS = new Set(['Read', 'Edit', 'Write', 'MultiEdit', 'NotebookEdit']);
@@ -557,13 +570,13 @@ export function parseTranscript(transcriptText, fallbackSessionId = null) {
   const ended_at = stamps[stamps.length - 1] || started_at;
   const durationMs = new Date(ended_at) - new Date(started_at);
 
-  const { skills, counts, errorCount } = collectToolUse(turn);
+  const { skills, counts, errors } = collectToolUse(turn);
 
   const events = [];
   for (const skill of skills) events.push({ kind: 'skill_invoked', skill });
   if (Object.keys(counts).length > 0) events.push({ kind: 'tool_summary', counts });
-  for (let i = 0; i < errorCount; i++) {
-    events.push({ kind: 'error', message: 'tool_result reported is_error' });
+  for (const err of errors) {
+    events.push({ kind: 'error', tool: err.tool, summary: err.summary });
   }
   events.push(...extractProcessEvents(turn, broken, total, durationMs));
   events.push(...extractAskUserQuestionEvents(turn));