feat(agent): auto-renew the host cert in the native run-loop (A5)

Wire createCertRotator/renewCert into superviseNative so the frp-client cert
renews silently at ~2/3 TTL over mTLS /renew and frpc restarts onto the fresh
leaf; failures retry with backoff, never crash the supervisor. mTLS transport
has a 15s timeout + 64KB response cap. 281 tests pass.

agent/test/frpSupervise.test.ts

@@ -145,4 +145,44 @@ describe('superviseFrpc (B4/H4 — restart-on-exit backoff)', () => {
     await stopping
     expect(spawn).toHaveBeenCalledTimes(1)
   })
+
+  it('restartChild() kills the running child so the loop respawns onto the fresh cert files', async () => {
+    const flush = (): Promise<void> => new Promise((r) => setImmediate(r))
+    const children = [makeChild(), makeChild()]
+    let n = 0
+    const spawn: SpawnFrpc = () => children[n++]!.child
+    const handle = superviseFrpc('/frpc', '/toml', {
+      spawn,
+      sleep: async () => {},
+      logger: silentLogger,
+      now: () => 1_000_000, // fixed clock: run counts as unstable, but sleep is a no-op → respawn is immediate
+    })
+    await flush()
+    expect(handle.isChildAlive()).toBe(true) // child[0]
+
+    handle.restartChild() // A5: rotator calls this after a cert rotation to reload the new leaf
+    expect(children[0]!.killed).toBe(true)
+
+    await flush()
+    expect(n).toBe(2) // child[1] spawned — frpc now reads the rotated cert on disk
+    expect(handle.isChildAlive()).toBe(true)
+    await handle.stop()
+  })
+
+  it('restartChild() after stop is a no-op (no spawn past shutdown)', async () => {
+    const first = makeChild()
+    let n = 0
+    const spawn: SpawnFrpc = vi.fn(() => (n++ === 0 ? first.child : makeChild().child))
+    const handle = superviseFrpc('/frpc', '/toml', {
+      spawn,
+      sleep: async () => {},
+      logger: silentLogger,
+    })
+    await Promise.resolve()
+    const stopping = handle.stop()
+    first.exit(0)
+    await stopping
+    handle.restartChild() // must not resurrect the supervisor
+    expect(spawn).toHaveBeenCalledTimes(1)
+  })
 })