fix(agent): make native cert auto-renew actually work end-to-end

Two real-deploy renew bugs (found running the live /renew):
- the /renew mTLS request pinned the enroll caChain as the server CA → TLS
  'unable to get local issuer certificate' (the LE-fronted CP is publicly
  trusted). Verify the server against SYSTEM roots (drop ca), keep the client
  cert + rejectUnauthorized:true. (TlsClientOptions.ca now optional.)
- renewCert parsed {cert, caChain:string}, but the CP returns cert=base64(DER)
  + caChain=base64(DER)[]; normalize to PEM (shared certs/pem.ts, reused by
  native enroll). Verified live: cert rotated 13:41→next-day, frpc restarted,
  tunnel stayed up. 281 tests pass.

agent/src/transport/dial.ts

@@ -25,7 +25,13 @@ export class CertExpiredError extends Error {
 export interface TlsClientOptions {
   readonly cert: string
   readonly key: string
-  readonly ca: string
+  /**
+   * CA(s) to verify the SERVER cert against. Set to the private tunnel CA for the relay dial; left
+   * undefined for a request whose server is publicly trusted (the LE-fronted control-plane /renew) so
+   * Node verifies against the system roots — pinning the private CA there fails with "unable to get
+   * local issuer certificate".
+   */
+  readonly ca?: string
   readonly rejectUnauthorized: true
 }