test(uok): fail-closed contract for triage-apply gate emission

Adds the missing test case that confirms the fail-closed semantics
the parallel worker shipped in slice 3a: when the trace writer
cannot persist a UOK gate record (e.g. .sf/traces is unwritable),
runTriageApply MUST abort before any subagent runs and surface the
emission failure as the run error.

This pins down the contract codex Q5 noted as soft: enrichment
failures are debug-only, but PRIMARY gate emission for the apply
flow is hard-required. Without observable gates, an apply that
mutates the ledger has no audit trail — refusing is the right call.

Test asserts: trace-dir write failure → ok=false, error contains
"UOK gate emission failed for trusted-agent-source-gate", and the
mocked agentRunner was never invoked.

Suite: 1682/1682.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/tests/headless-triage-uok-gates.test.ts

@@ -8,7 +8,14 @@
  * surface=headless, runControl=supervised, permissionProfile=high, traceId=
  * the flowId. The outcome reflects the decision (pass/fail/manual-attention).
  */
-import { mkdirSync, mkdtempSync, readFileSync, readdirSync, rmSync } from "node:fs";
+import {
+	mkdirSync,
+	mkdtempSync,
+	readFileSync,
+	readdirSync,
+	rmSync,
+	writeFileSync,
+} from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { afterEach, beforeEach, describe, expect, test } from "vitest";
@@ -179,4 +186,25 @@ describe("runTriageApply emits gate_run trace events with schema-v2 metadata", (
 		// Review gate must NOT fire because plan validation blocked the flow.
 		expect(reviewEvents).toHaveLength(0);
 	});
+
+	test("trace_write_failure_fails_closed_before_agents_run", async () => {
+		const project = makeProject();
+		writeFileSync(join(project, ".sf", "traces"), "not a directory");
+		let calls = 0;
+
+		const result = await runTriageApply(project, "triage prompt", {
+			candidateCount: 1,
+			allowUntrustedRunner: true,
+			agentRunner: async () => {
+				calls += 1;
+				return { ok: true, output: deciderPlan, exitCode: 0 };
+			},
+		});
+
+		expect(result.ok).toBe(false);
+		expect(result.error).toContain(
+			"UOK gate emission failed for trusted-agent-source-gate",
+		);
+		expect(calls).toBe(0);
+	});
 });