feat(caveman): add targeted upstream-aligned style rules (#2424)

Add four shared Caveman prompt fragments (no invented abbreviations,
preserve user language, no self-reference, no decoration) across all six
levels, and remove ULTRA contradictions around abbreviations/arrow
shorthand. Adds regression tests for the prompt rules.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/caveman-prompts.test.js

@@ -0,0 +1,79 @@
+import { describe, it, expect } from "vitest";
+import { CAVEMAN_LEVELS, CAVEMAN_PROMPTS } from "../../open-sse/rtk/cavemanPrompts.js";
+
+const LEVEL_KEYS = [
+  CAVEMAN_LEVELS.LITE,
+  CAVEMAN_LEVELS.FULL,
+  CAVEMAN_LEVELS.ULTRA,
+  CAVEMAN_LEVELS.WENYAN_LITE,
+  CAVEMAN_LEVELS.WENYAN,
+  CAVEMAN_LEVELS.WENYAN_ULTRA,
+];
+
+describe("Caveman prompt coverage", () => {
+  it("every level key has matching prompt and vice versa", () => {
+    const levelValues = Object.values(CAVEMAN_LEVELS);
+    for (const key of LEVEL_KEYS) {
+      expect(levelValues).toContain(key);
+    }
+    for (const value of levelValues) {
+      expect(LEVEL_KEYS).toContain(value);
+    }
+  });
+
+  it("has a prompt string for every level", () => {
+    for (const level of LEVEL_KEYS) {
+      expect(typeof CAVEMAN_PROMPTS[level]).toBe("string");
+      expect(CAVEMAN_PROMPTS[level].length).toBeGreaterThan(0);
+    }
+  });
+
+  it("adds no-invented-abbreviations guidance to every level", () => {
+    for (const level of LEVEL_KEYS) {
+      expect(CAVEMAN_PROMPTS[level]).toContain("No invented abbreviations");
+    }
+  });
+
+  it("adds preserve-user-language guidance to every level", () => {
+    for (const level of LEVEL_KEYS) {
+      expect(CAVEMAN_PROMPTS[level]).toContain("Preserve the user's dominant language");
+    }
+  });
+
+  it("adds no-self-reference guidance to every level", () => {
+    for (const level of LEVEL_KEYS) {
+      expect(CAVEMAN_PROMPTS[level]).toContain("No self-reference");
+    }
+  });
+
+  it("adds no-decorative-emoji guidance to every level", () => {
+    for (const level of LEVEL_KEYS) {
+      expect(CAVEMAN_PROMPTS[level]).toContain("No decorative emoji");
+    }
+  });
+});
+
+describe("Caveman internal consistency", () => {
+  it("no level uses Unicode arrow (SHARED_NO_DECORATION bans arrow shorthand)", () => {
+    // SHARED_NO_DECORATION uses ASCII -> to quote the banned pattern.
+    // Unicode → is the character old ULTRA used in "Pattern: [thing] → [result]".
+    // Verify no level now uses it.
+    for (const level of LEVEL_KEYS) {
+      expect(CAVEMAN_PROMPTS[level]).not.toContain("→");
+    }
+  });
+});
+
+describe("Caveman ULTRA targeted sync", () => {
+  it("does not encourage invented abbreviations", () => {
+    const ultra = CAVEMAN_PROMPTS[CAVEMAN_LEVELS.ULTRA];
+    expect(ultra).not.toContain("req/res/fn/impl");
+    expect(ultra).not.toContain("Abbreviate (DB/auth/config/req/res/fn/impl)");
+  });
+
+  it("does not encourage arrow shorthand", () => {
+    const ultra = CAVEMAN_PROMPTS[CAVEMAN_LEVELS.ULTRA];
+    expect(ultra).not.toContain("use arrows for causality");
+    expect(ultra).not.toContain("X → Y");
+  });
+});