feat(tag-vocab): prompt.ts — PROMPT_VERSION 4 + vocab parameter (#3 v0.2.3)

- PROMPT_VERSION 3 → 4 (marker bump, retry 트리거 X)
- buildPrompt 4번째 param vocab: string[] = []
- vocab.length > 0 시 "Existing vocabulary tags" + "Prefer reusing" 라인 추가
- vocab=[] 시 라인 자체 생략 (Q3=B 결정)
- 단위 +4 cases (신규 prompt.test.ts)

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

tests/unit/prompt.test.ts

@@ -0,0 +1,31 @@
+import { describe, it, expect } from 'vitest';
+import { buildPrompt, PROMPT_VERSION } from '@main/ai/prompt.js';
+
+describe('prompt', () => {
+  it('PROMPT_VERSION is 4', () => {
+    expect(PROMPT_VERSION).toBe(4);
+  });
+
+  it('buildPrompt with empty vocab omits vocabulary line entirely', () => {
+    const out = buildPrompt('hello', '2026-05-02', [], []);
+    expect(out).not.toContain('vocabulary');
+    expect(out).not.toContain('Prefer reusing');
+  });
+
+  it('buildPrompt with vocab includes Prefer instruction + comma-separated list', () => {
+    const out = buildPrompt('hello', '2026-05-02', [], ['design', 'meeting', 'qa']);
+    expect(out).toContain('Existing vocabulary tags');
+    expect(out).toContain('design, meeting, qa');
+    expect(out).toContain('Prefer reusing');
+  });
+
+  it('vocab block appears between candidate block and JSON rules', () => {
+    const out = buildPrompt('hello', '2026-05-02', [], ['design']);
+    const candidateIdx = out.indexOf("Today's date");
+    const vocabIdx = out.indexOf('Existing vocabulary');
+    const jsonRulesIdx = out.indexOf('Return a JSON object');
+    expect(candidateIdx).toBeGreaterThan(-1);
+    expect(vocabIdx).toBeGreaterThan(candidateIdx);
+    expect(jsonRulesIdx).toBeGreaterThan(vocabIdx);
+  });
+});