feat(observer): Task 2 — extractTokenUsage + task_cost in parseTranscript

- export extractTokenUsage(turn): sums input/output/cache/iterations/
  web_search/web_fetch across all assistant messages in a turn
- parseTranscript now includes task_cost field (zero-filled when no usage)
- 7 new tests (5 unit + 2 integration); total 248/248 GREEN
- V2_FIELDS in observer-stop-hook.mjs NOT changed (backward compat)

tools/observer-transcript-parser.test.mjs

@@ -8,6 +8,7 @@ import {
   parseRoutingTag,
   extractLastUserPromptText,
   classifyTask,
+  extractTokenUsage,
 } from './observer-transcript-parser.mjs';
 
 // Build a JSONL transcript string from entry objects.
@@ -920,3 +921,85 @@ describe('classifyTask — extended dictionary (Task 1)', () => {
     expect(classifyTask('почини баг в logger')).toBe('bugfix');
   });
 });
+
+describe('extractTokenUsage (Task 2)', () => {
+  it('sums input/output/cache fields across multiple assistant messages', () => {
+    const turn = [
+      { message: { usage: { input_tokens: 10, output_tokens: 5, cache_read_input_tokens: 100, cache_creation_input_tokens: 50 } } },
+      { message: { usage: { input_tokens: 8, output_tokens: 3, cache_read_input_tokens: 80, cache_creation_input_tokens: 20 } } },
+    ];
+    expect(extractTokenUsage(turn)).toEqual({
+      input_tokens: 18, output_tokens: 8, cache_read_input_tokens: 180,
+      cache_creation_input_tokens: 70, web_search_requests: 0, web_fetch_requests: 0, iterations: 0,
+    });
+  });
+  it('captures server_tool_use bonus fields (web_search/web_fetch)', () => {
+    const turn = [
+      { message: { usage: { input_tokens: 5, output_tokens: 2, server_tool_use: { web_search_requests: 3, web_fetch_requests: 1 } } } },
+    ];
+    const result = extractTokenUsage(turn);
+    expect(result.web_search_requests).toBe(3);
+    expect(result.web_fetch_requests).toBe(1);
+  });
+  it('captures iterations (extended-thinking detector)', () => {
+    const turn = [
+      { message: { usage: { input_tokens: 100, output_tokens: 50, iterations: 4 } } },
+    ];
+    expect(extractTokenUsage(turn).iterations).toBe(4);
+  });
+  it('returns zero-filled object when no usage present', () => {
+    const turn = [
+      { message: {} },
+      { message: { usage: null } },
+    ];
+    expect(extractTokenUsage(turn)).toEqual({
+      input_tokens: 0, output_tokens: 0, cache_read_input_tokens: 0,
+      cache_creation_input_tokens: 0, web_search_requests: 0, web_fetch_requests: 0, iterations: 0,
+    });
+  });
+  it('handles empty/null turn safely', () => {
+    expect(extractTokenUsage([])).toEqual({
+      input_tokens: 0, output_tokens: 0, cache_read_input_tokens: 0,
+      cache_creation_input_tokens: 0, web_search_requests: 0, web_fetch_requests: 0, iterations: 0,
+    });
+    expect(extractTokenUsage(null)).toEqual({
+      input_tokens: 0, output_tokens: 0, cache_read_input_tokens: 0,
+      cache_creation_input_tokens: 0, web_search_requests: 0, web_fetch_requests: 0, iterations: 0,
+    });
+  });
+  it('safely skips entries where usage is a non-object primitive (defensive guard)', () => {
+    const turn = [
+      { message: { usage: 42 } }, // malformed — usage as primitive
+      { message: { usage: { input_tokens: 5, output_tokens: 3 } } },
+    ];
+    const r = extractTokenUsage(turn);
+    expect(r.input_tokens).toBe(5);
+    expect(r.output_tokens).toBe(3);
+  });
+});
+
+describe('parseTranscript — task_cost integration (Task 2)', () => {
+  it('attaches task_cost to a v2 episode', () => {
+    const lines = [
+      JSON.stringify({ type: 'user', message: { role: 'user', content: [{ type: 'text', text: 'implement feature X' }] } }),
+      JSON.stringify({ type: 'assistant', message: { role: 'assistant', content: [{ type: 'text', text: 'done' }], usage: { input_tokens: 42, output_tokens: 7 } } }),
+    ];
+    const result = parseTranscript(lines.join('\n'));
+    expect(result).not.toBeNull();
+    expect(result.task_cost).toBeDefined();
+    expect(result.task_cost.input_tokens).toBe(42);
+    expect(result.task_cost.output_tokens).toBe(7);
+  });
+  it('attaches zero-filled task_cost when no usage in transcript', () => {
+    const lines = [
+      JSON.stringify({ type: 'user', message: { role: 'user', content: [{ type: 'text', text: 'do something' }] } }),
+      JSON.stringify({ type: 'assistant', message: { role: 'assistant', content: [{ type: 'text', text: 'ok' }] } }),
+    ];
+    const result = parseTranscript(lines.join('\n'));
+    expect(result).not.toBeNull();
+    expect(result.task_cost).toEqual({
+      input_tokens: 0, output_tokens: 0, cache_read_input_tokens: 0,
+      cache_creation_input_tokens: 0, web_search_requests: 0, web_fetch_requests: 0, iterations: 0,
+    });
+  });
+});