chore(client): mark all client tasks as completed

Phase 3 complete: alknet-client crate implemented with:
- AlknetClient struct with builder methods (with_quinn, with_tcp_tls, with_iroh, with_socks5_proxy)
- ClientDialError enum with 5 variants
- dial_quic, dial_tcp_tls, dial_iroh methods
- SOCKS5 proxy support (Socks5ProxyConfig, Socks5Credentials, Socks5UdpSocket)
- Unit tests (18) + integration test (3)
- All feature combos compile and test clean
- Workspace build and tests pass

tasks/client/client-core.md

@@ -1,7 +1,7 @@
 ---
 id: client/client-core
 name: Implement AlknetClient struct, new, and builder methods (with_quinn, with_tcp_tls, with_iroh, with_socks5_proxy)
-status: pending
+status: completed
 depends_on: [client/error-type]
 scope: moderate
 risk: medium