Add mTLS to CP→agent channel

- Internal ECDSA P-256 CA (WRENN_CA_CERT/WRENN_CA_KEY env vars); when absent
  the system falls back to plain HTTP so dev mode works without certificates
- Host leaf cert (7-day TTL, IP SAN) issued at registration and renewed on
  every JWT refresh; fingerprint + expiry stored in DB (cert_expires_at column
  replaces the removed mtls_enabled flag)
- CP ephemeral client cert (24-hour TTL) via CPCertStore with atomic hot-swap;
  background goroutine renews it every 12 hours without restarting the server
- Host agent uses tls.Listen + httpServer.Serve so GetCertificate callback is
  respected (ListenAndServeTLS always reads cert from disk)
- Sandbox reverse proxy now uses pool.Transport() so it shares the same TLS
  config as the Connect RPC clients instead of http.DefaultTransport
- Credentials file renamed host-credentials.json with cert_pem/key_pem/
  ca_cert_pem fields; duplicate register/refresh response structs collapsed
  to authResponse

.env.example

@@ -27,6 +27,14 @@ AWS_SECRET_ACCESS_KEY=
 # Auth
 JWT_SECRET=
 
+# mTLS — CP→Agent channel
+# Generate a self-signed CA with:
+#   openssl ecparam -genkey -name P-256 -noout -out ca.key
+#   openssl req -new -x509 -key ca.key -days 3650 -out ca.crt -subj "/CN=wrenn-internal-ca"
+# Then set these to the file contents (newlines replaced with \n or use multiline env).
+WRENN_CA_CERT=-----BEGIN CERTIFICATE-----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\nVM7hllXxL6AdxJtU2vsEoA0=\n-----END CERTIFICATE-----
+WRENN_CA_KEY=-----BEGIN EC PRIVATE KEY-----\nMHcCAQEEIOjpTSFMhhR9Yi2mWtrzJ/FINEImtmz32GkwZ9eYUbDkoAoGCCqGSM49\nAwEHoUQDQgAEOTC/xrVjtfHtrnJQMvANRQGfV9J9SWrqwatWhyJLZlr6+jXfncrr\nUgUw0X0DfC8temwQAZkptrDWIR9GzosRGA==\n-----END EC PRIVATE KEY-----
+
 # OAuth
 OAUTH_GITHUB_CLIENT_ID=
 OAUTH_GITHUB_CLIENT_SECRET=