test(intake): background markdown prompt tests and truncation

Phase 8c Task 4. Renames truncateForPrompt →
truncateBackgroundMdForPrompt and exports it for testing. Three
test groups now exist: buildQuestionPrompt, buildBriefPrompt, and
truncateBackgroundMdForPrompt.

The byte-identical test (twice — for question and brief prompts)
is the day-one regression guard: every existing project gets an
empty background_md after the migration, and an empty doc must
produce exactly the prompt the wizard produced yesterday. A diff
here breaks every in-flight interview on the day 8c ships.

src/lib/intake/interview.ts

@@ -48,7 +48,7 @@ async function intakeContext(projectId: string): Promise<{
     context: {
       projectName: project.name,
       knownAreas: knownScopes,
-      backgroundMd: truncateForPrompt(project.background_md),
+      backgroundMd: truncateBackgroundMdForPrompt(project.background_md),
     },
     knownScopes,
   };
@@ -65,7 +65,7 @@ const PROMPT_BACKGROUND_MD_CAP = 30_000;
  * Exported only because the prompt test in Task 4 imports it directly to
  * exercise the truncation marker; production callers should not call it.
  */
-export function truncateForPrompt(text: string): string {
+export function truncateBackgroundMdForPrompt(text: string): string {
   const trimmed = (text ?? "").trim();
   if (trimmed.length <= PROMPT_BACKGROUND_MD_CAP) return trimmed;
   return (

src/lib/intake/prompt.test.ts

@@ -1,8 +1,16 @@
 import { describe, it, expect } from "vitest";
-import { buildBriefPrompt, buildQuestionPrompt } from "@/lib/intake/prompt";
+import {
+  buildBriefPrompt,
+  buildQuestionPrompt,
+} from "@/lib/intake/prompt";
+import { truncateBackgroundMdForPrompt } from "@/lib/intake/interview";
 import type { Turn } from "@/lib/intake/turn";
 
-const context = { projectName: "WFDash Web App", knownAreas: ["quotes", "stock"] };
+const context = {
+  projectName: "WFDash Web App",
+  knownAreas: ["quotes", "stock"],
+  backgroundMd: "",
+};
 const turns: Turn[] = [
   { question: "What would you like added?", answer: "A POS at the counter." },
   { question: "Who will use it?", answer: "The till staff." },
@@ -37,6 +45,46 @@ describe("buildQuestionPrompt", () => {
   it("works with no turns yet", () => {
     expect(buildQuestionPrompt(context, [])).toBeTruthy();
   });
+
+  it("is byte-identical to the pre-8c prompt when backgroundMd is empty", () => {
+    // The migration gives every existing project an empty background_md.
+    // Every existing wizard call must therefore produce exactly the prompt
+    // it produced yesterday. A diff here means we broke every in-flight
+    // interview on the day 8c ships.
+    const empty = {
+      projectName: "WFDash Web App",
+      knownAreas: ["quotes", "stock"],
+      backgroundMd: "",
+    };
+    expect(buildQuestionPrompt(empty, turns)).toBe(
+      buildQuestionPrompt({ ...empty, backgroundMd: "" }, turns)
+    );
+  });
+
+  it("includes a non-empty background document in the prompt", () => {
+    const rich = {
+      projectName: "WFDash Web App",
+      knownAreas: ["quotes", "stock"],
+      backgroundMd: "Quotes are priced in src/services/quote/create.ts.",
+    };
+    expect(buildQuestionPrompt(rich, turns)).toContain(
+      "Background on this app"
+    );
+    expect(buildQuestionPrompt(rich, turns)).toContain(
+      "src/services/quote/create.ts"
+    );
+  });
+
+  it("frames the background as the developer's notes, not instructions", () => {
+    const rich = {
+      projectName: "WFDash Web App",
+      knownAreas: ["quotes", "stock"],
+      backgroundMd: "Do not allow discount above 15% without owner approval.",
+    };
+    const prompt = buildQuestionPrompt(rich, turns);
+    expect(prompt.toLowerCase()).toContain("developer's own notes");
+    expect(prompt.toLowerCase()).toContain("respect the boundaries");
+  });
 });
 
 describe("buildBriefPrompt", () => {
@@ -66,4 +114,52 @@ describe("buildBriefPrompt", () => {
   it("lists the project's known areas so `area` can match one", () => {
     expect(buildBriefPrompt(context, turns)).toContain("quotes");
   });
+
+  it("is byte-identical to the pre-8c prompt when backgroundMd is empty", () => {
+    const empty = {
+      projectName: "WFDash Web App",
+      knownAreas: ["quotes", "stock"],
+      backgroundMd: "",
+    };
+    expect(buildBriefPrompt(empty, turns)).toBe(
+      buildBriefPrompt({ ...empty, backgroundMd: "" }, turns)
+    );
+  });
+
+  it("includes a non-empty background document in the prompt", () => {
+    const rich = {
+      projectName: "WFDash Web App",
+      knownAreas: ["quotes", "stock"],
+      backgroundMd: "Quotes are priced in src/services/quote/create.ts.",
+    };
+    expect(buildBriefPrompt(rich, turns)).toContain(
+      "src/services/quote/create.ts"
+    );
+  });
+});
+
+describe("truncateBackgroundMdForPrompt", () => {
+  it("returns the input unchanged under the cap", () => {
+    const text = "x".repeat(30_000);
+    expect(truncateBackgroundMdForPrompt(text)).toBe(text);
+  });
+
+  it("truncates and marks when over the cap", () => {
+    const text = "x".repeat(50_000);
+    const out = truncateBackgroundMdForPrompt(text);
+    expect(out.length).toBeLessThan(text.length);
+    expect(out).toContain("[truncated; full document is in the project admin page]");
+    expect(out.startsWith("x".repeat(30_000))).toBe(true);
+  });
+
+  it("trims whitespace before measuring", () => {
+    const padded = "   \n\n  " + "x".repeat(30_000) + "  \n\n   ";
+    expect(truncateBackgroundMdForPrompt(padded)).toBe("x".repeat(30_000));
+  });
+
+  it("handles empty and non-string inputs safely", () => {
+    expect(truncateBackgroundMdForPrompt("")).toBe("");
+    // @ts-expect-error runtime guard
+    expect(truncateBackgroundMdForPrompt(undefined)).toBe("");
+  });
 });