feat(observer): opt-in reasoning-tag merges with heuristic primary_rationale

Closes brain-retro 2026-05-20 #11 — parseReasoningTag extracts opt-in
<!-- reasoning: triggers="..." candidates="..." boundaries="..." -->
HTML-comment from assistant text. Semicolon-separated values merged into
heuristic-derived primary_rationale arrays via Set-dedupe.

Conservative: tag is opt-in; heuristic still runs even when tag present
(heuristic provides baseline, tag enriches).

5 new vitest tests, 309/309 GREEN.

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

tools/observer-transcript-parser.test.mjs

@@ -1327,3 +1327,51 @@ describe('classifyPromptSignal — extended dictionary (Task 9)', () => {
     expect(classifyPromptSignal('переделай это')).toBe('correction');
   });
 });
+
+import { parseReasoningTag } from './observer-transcript-parser.mjs';
+
+describe('parseReasoningTag (Task 11)', () => {
+  it('parses opt-in reasoning tag from assistant text', () => {
+    const turn = [{ message: { role: 'assistant', content: [
+      { type: 'text', text: '<!-- reasoning: triggers="Pravila §12.2;ADR-011" candidates="brain-retro;brainstorming" boundaries="Pravila §16" -->\nAnswer.' }
+    ] } }];
+    const tag = parseReasoningTag(turn);
+    expect(tag).toEqual({
+      triggers: ['Pravila §12.2', 'ADR-011'],
+      candidates: ['brain-retro', 'brainstorming'],
+      boundaries: ['Pravila §16'],
+    });
+  });
+  it('returns null when no tag present', () => {
+    expect(parseReasoningTag([{ message: { role: 'assistant', content: [{ type: 'text', text: 'plain' }] } }])).toBeNull();
+  });
+  it('safe on null/empty turn', () => {
+    expect(parseReasoningTag(null)).toBeNull();
+    expect(parseReasoningTag([])).toBeNull();
+  });
+  it('skips non-text blocks', () => {
+    const turn = [{ message: { role: 'assistant', content: [{ type: 'tool_use', name: 'Read' }] } }];
+    expect(parseReasoningTag(turn)).toBeNull();
+  });
+});
+
+describe('parseTranscript — reasoning-tag merges with heuristic (Task 11)', () => {
+  it('merges tag triggers into heuristic triggers (deduped)', () => {
+    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: 'text', text: 'Pravila §12.2 hard-rule\n<!-- reasoning: triggers="Pravila §12.2;ADR-011" candidates="brain-retro" boundaries="Pravila §16" -->' }
+      ] }, uuid: 'u2', timestamp: '2026-05-20T00:01:00Z' }),
+    ].join('\n');
+    const ep = parseTranscript(transcript);
+    // heuristic finds 'Pravila §12.2' from text + tag adds 'ADR-011'; dedup
+    expect(ep.primary_rationale.triggers_matched).toContain('Pravila §12.2');
+    expect(ep.primary_rationale.triggers_matched).toContain('ADR-011');
+    expect(ep.primary_rationale.triggers_matched.filter((t) => t === 'Pravila §12.2')).toHaveLength(1);
+    // candidates: tag contributes 'brain-retro' (no numbered list in text)
+    expect(ep.primary_rationale.candidates_considered).toContain('brain-retro');
+    // boundaries: heuristic empty (no marker in plain text), tag adds 'Pravila §16'
+    expect(ep.primary_rationale.boundaries_applied).toContain('Pravila §16');
+  });
+});