test: harness-mode + unattended-tier coverage

[arthurpanhku]

Why

src/harness/run.ts (executeHarnessRun) is the headless entrypoint (docs/HARNESS-MODE.md) and is currently only exercised indirectly via tests/mcp/server.test.ts and tests/commands/run.test.ts. There's no tests/harness/ and, critically, no dedicated test pinning unattended-tier behavior — the most governance-sensitive path, since it runs without a human in the loop.

Sketch

• tests/harness/run.test.ts driving executeHarnessRun directly: happy path, policy-denied tool, and approval-required tool under each UnattendedPermissionMode.
• A bypass-proof assertion: under the unattended tier, an action that policy forbids is refused and audited — it must not silently auto-approve.
• Assert the run still produces a verifiable audit chain (run_start … tamper-evident) in headless mode.

Acceptance

• Changing unattended/approval semantics can't regress without a red test.
• npm run check covers the harness path directly, not just transitively.