feat(FN-1429): enforce always-green test-suite instructions for executor agents

- Add explicit always-green test suite instructions to executor agent prompts
- Update executor to enforce test-suite validation before task completion
- Add tests for agent prompt generation and executor behavior
- Ensure test failures block merge-ready state rather than allowing broken builds

packages/core/src/agent-prompts.test.ts

@@ -123,6 +123,27 @@ describe("resolveAgentPrompt", () => {
     const result = resolveAgentPrompt("executor", config);
     expect(result).toContain("task execution agent");
   });
+
+  it("built-in executor prompt requires resolving ALL test failures including unrelated", () => {
+    const result = resolveAgentPrompt("executor");
+    // The stricter language must be present to prevent "unrelated failure" deferrals
+    expect(result).toContain("Resolve ALL test failures");
+    expect(result).toContain("even if they appear unrelated or pre-existing");
+    expect(result).toContain("do not defer them to a separate task");
+  });
+
+  it("senior-engineer prompt requires resolving ALL test failures including unrelated", () => {
+    const config: AgentPromptsConfig = {
+      roleAssignments: {
+        executor: "senior-engineer",
+      },
+    };
+
+    const result = resolveAgentPrompt("executor", config);
+    expect(result).toContain("Resolve ALL test failures");
+    expect(result).toContain("even if they appear unrelated or pre-existing");
+    expect(result).toContain("do not defer them to a separate task");
+  });
 });
 
 // ---------------------------------------------------------------------------

packages/core/src/agent-prompts.ts

@@ -141,7 +141,8 @@ If a project build command is listed in the prompt, it is a hard completion gate
 Tests and typecheck are also hard quality gates:
 - Keep fixing failures until the configured/full test suite passes
 - If the repository exposes a typecheck command, run it and keep fixing failures until it passes
-- Do not stop at "out of scope" if additional fixes are required to restore green tests, build, or typecheck`;
+- Do not stop at "out of scope" if additional fixes are required to restore green tests, build, or typecheck
+- **CRITICAL: Resolve ALL test failures before completing the task, even if they appear unrelated or pre-existing.** Unrelated failures left unfixed accumulate technical debt and block future integrations. Investigate and fix or suppress them — do not defer them to a separate task.`;
 
 const TRIAGE_PROMPT_TEXT = `You are a task specification agent for "fn", an AI-orchestrated task board.
 
@@ -494,7 +495,8 @@ Call \`task_done()\` to signal completion.
 \`\`\`
 
 If a project build command is listed in the prompt, it is a hard completion gate.
-Tests and typecheck are also hard quality gates — keep fixing until green.`;
+Tests and typecheck are also hard quality gates — keep fixing until green.
+**CRITICAL: Resolve ALL test failures before completing the task, even if they appear unrelated or pre-existing.** Unrelated failures left unfixed accumulate technical debt and block future integrations. Investigate and fix or suppress them — do not defer them to a separate task.`;
 
 const STRICT_REVIEWER_PROMPT_TEXT = `You are a strict code and plan reviewer with rigorous standards.