fix(coding-agent): keep smoke-test fallback for uncovered UI surfaces

When a session has browser but not computer (or vice versa), the
behavioral/smoke-test fallback for UI changes was suppressed entirely,
leaving native-desktop (or web) verification guidance undefined. Render
the fallback whenever any visual runtime tool is missing.

Also strengthen the plan-mode tests per review: exercise the iterative
branch gating with iterative:true (previously passed trivially) and
restore a typed Overrides record for the render helper.

packages/coding-agent/test/system-prompt-inventory.test.ts

@@ -724,6 +724,9 @@ describe("system prompt tool inventory", () => {
 		).systemPrompt.join("\n\n");
 
 		expect(withBrowser).toContain("drive it in `browser`");
+		// A browser-only session still needs the smoke-test fallback for
+		// native-desktop surfaces (no computer tool).
+		expect(withBrowser).toContain("behavioral test or smoke test");
 	});
 
 	it("omits todo workflow guidance when the todo tool is absent", async () => {