C4 follow-up #4: add message_does_not_contain_substring assertion kind + pin C4 invariants

Closes the fourth ask from the C4 cross-review (docs/proposals/C4-followup.md
Item 4). The eval harness previously had no way to assert the absence of a
pattern in a trace — only positive `message_contains_substring`. That meant
the C4 "no more node -e / bash+fetch from CEO" invariant was only
demonstrated by the diff between two commits, not by any running assertion
that future regressions would trip.

Three changes:

1. Extend the Assertion union in tests/eval/types.ts with
   `message_does_not_contain_substring` (role + substring). Symmetric to
   the positive kind; fails on first match with a specific reason.

2. Add the case in assertion-judge.ts. Same traversal as the positive
   branch but inverted exit condition.

3. Add two harness self-tests covering pass + fail, so the new kind is
   exercised even when the fixtures directory is empty.

4. Pin two C4-regression assertions onto the recorded ceo-bootstrap-empty
   fixture: the assistant response must NOT contain "node -e" OR
   "/api/companies". Any future regression that reintroduces the
   HTTP-path-in-prompt pattern or the bash+node -e shell fallback will
   now fail pnpm test:eval instead of living in the fixture silently.

typecheck green, test:eval up from 8 → 10 tests (2 new self-tests), all
passing.

tests/eval/harness.test.ts

@@ -131,6 +131,34 @@ describe("harness self-test (in-memory fixture)", () => {
     expect(reasons[2]).toContain("trace has 2 turns, max allowed is 1");
   });
 
+  it("message_does_not_contain_substring passes when substring is absent", () => {
+    const fixture = buildInMemoryFixture([
+      {
+        kind: "message_does_not_contain_substring",
+        role: "assistant",
+        substring: "node -e",
+      },
+    ]);
+    const result = runEval(fixture);
+    expect(result.passed).toBe(true);
+    expect(result.assertions[0]?.passed).toBe(true);
+  });
+
+  it("message_does_not_contain_substring fails when substring is present", () => {
+    const fixture = buildInMemoryFixture([
+      {
+        kind: "message_does_not_contain_substring",
+        role: "assistant",
+        substring: "Hiring a writer",
+      },
+    ]);
+    const result = runEval(fixture);
+    expect(result.passed).toBe(false);
+    expect(result.assertions[0]?.reason).toContain(
+      'a assistant message contained forbidden substring "Hiring a writer"',
+    );
+  });
+
   it("judgeAssertions is a pure function of (assertions, turns)", () => {
     const fixture = buildInMemoryFixture([]);
     const first = judgeAssertions(