FN-5893: require invariant-level bug regression coverage

Add invariant-first regression guidance to triage prompts and testing docs.

- add an AGENTS standing rule to cover bug-fix invariants across all known surfaces
- update standard, fast, and core triage prompt templates to require invariant-level regression tests
- add triage regression tests that lock the new wording into standard, fast, and core prompts
- link docs/testing guidance back to the new invariant-over-repro rule

Files changed:
 AGENTS.md                                    |  7 ++++++
 docs/testing.md                              |  1 +
 packages/core/src/agent-prompts.ts           |  3 ++-
 packages/engine/src/__tests__/triage.test.ts | 32 ++++++++++++++++++++++++++++
 packages/engine/src/triage.ts                |  6 ++++--
 5 files changed, 46 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-5893

Fusion-Task-Lineage: 31c082c6-2a0a-4619-a13b-94dad97eef62

packages/engine/src/__tests__/triage.test.ts

@@ -9,9 +9,11 @@ import {
   computeUserCommentFingerprint,
 } from "../triage.js";
 import { join } from "node:path";
+import { readFileSync } from "node:fs";
 import { mkdir, writeFile, rm, mkdtemp } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { setTimeout as delay } from "node:timers/promises";
+import { fileURLToPath } from "node:url";
 import { planLog } from "../logger.js";
 
 const { mockReviewStep, mockCreateFnAgent } = vi.hoisted(() => ({
@@ -645,6 +647,36 @@ describe("TRIAGE_SYSTEM_PROMPT", () => {
   });
 });
 
+describe("FN-5893 invariant regression wording", () => {
+  it("requires invariant-level regression coverage in standard, fast, and core triage prompts", () => {
+    const corePromptSource = readFileSync(
+      fileURLToPath(new URL("../../../core/src/agent-prompts.ts", import.meta.url)),
+      "utf8",
+    );
+
+    for (const prompt of [
+      TRIAGE_SYSTEM_PROMPT,
+      FAST_TRIAGE_SYSTEM_PROMPT,
+      corePromptSource,
+    ]) {
+      expect(prompt).toContain("invariant across all known surfaces");
+      expect(prompt).toContain("provider/bridge");
+      expect(prompt).toContain("desktop + mobile breakpoints");
+      expect(prompt).toContain("empty/undefined/populated data states");
+      expect(prompt).toContain("FN-5787/FN-5789/FN-5803");
+      expect(prompt).toContain("FN-5751");
+    }
+  });
+
+  it("requires implementation-step testing guidance to enumerate invariant surfaces in standard and fast prompts", () => {
+    for (const prompt of [TRIAGE_SYSTEM_PROMPT, FAST_TRIAGE_SYSTEM_PROMPT]) {
+      expect(prompt).toContain(
+        "Run targeted tests for changed files, asserting the invariant across all known surfaces",
+      );
+    }
+  });
+});
+
 describe("fast-mode triage", () => {
   it("exports a lean FAST_TRIAGE_SYSTEM_PROMPT", () => {
     expect(typeof FAST_TRIAGE_SYSTEM_PROMPT).toBe("string");

packages/engine/src/triage.ts

@@ -144,7 +144,7 @@ Follow this structure exactly:
 
 - [ ] {Specific, verifiable outcome}
 - [ ] {Specific, verifiable outcome}
-- [ ] Run targeted tests for changed files
+- [ ] Run targeted tests for changed files, asserting the invariant across all known surfaces (enumerate every provider/bridge, desktop + mobile breakpoints, and empty/undefined/populated data states)
 
 **Artifacts:**
 - \`path/to/file\` (new | modified)
@@ -220,6 +220,7 @@ files with assertions that run via a test runner. Typechecks and builds are NOT
 tests. Manual verification is NOT a test.
 
 - Each implementation step should include writing tests for the code being changed
+- For bug fixes, regression tests must assert the invariant across all known surfaces — enumerate every provider/bridge, desktop + mobile breakpoints, and empty/undefined/populated data states — not just the reported repro (see FN-5787/FN-5789/FN-5803 and FN-5751)
 - The final Testing step runs lint, the FULL test suite, and project typecheck when the repo exposes one
 - Specs must instruct executors to fix lint failures and quality-gate failures directly, even when the required edits extend beyond the original File Scope
 - If the project has no test framework, the Testing step must include setting one up
@@ -429,7 +430,7 @@ Follow this structure exactly:
 
 - [ ] {Specific, verifiable outcome}
 - [ ] {Specific, verifiable outcome}
-- [ ] Run targeted tests for changed files
+- [ ] Run targeted tests for changed files, asserting the invariant across all known surfaces (enumerate every provider/bridge, desktop + mobile breakpoints, and empty/undefined/populated data states)
 
 **Artifacts:**
 - \`path/to/file\` (new | modified)
@@ -500,6 +501,7 @@ If this task REMOVES existing functionality (deleting modules, settings, API end
 ## Testing requirements
 - Require real automated tests with assertions that run in the project's test runner
 - Typecheck/build/manual checks are not tests and cannot replace tests
+- For bug fixes, regression tests must assert the invariant across all known surfaces — enumerate every provider/bridge, desktop + mobile breakpoints, and empty/undefined/populated data states — not just the reported repro (see FN-5787/FN-5789/FN-5803 and FN-5751)
 - Include targeted tests in implementation steps and full quality-gate runs in final verification
 
 ## Duplicate check