feat: add Docker env var support for PORT, HOST, and TEMPERATURE

- Add port and host fields to GatewayConfig with defaults (3000, 127.0.0.1)
- Enhanced apply_env_overrides() to support:
  - ZEROCLAW_GATEWAY_PORT or PORT - Gateway server port
  - ZEROCLAW_GATEWAY_HOST or HOST - Gateway bind address
  - ZEROCLAW_TEMPERATURE - Default temperature (0.0-2.0)
- Add comprehensive tests for all new env var overrides
- Fix clippy warnings (is_multiple_of, too_many_lines)

Closes #45

src/security/secrets.rs

@@ -241,7 +241,7 @@ fn hex_encode(data: &[u8]) -> String {
 
 /// Hex-decode a hex string to bytes.
 fn hex_decode(hex: &str) -> Result<Vec<u8>> {
-    if hex.len() % 2 != 0 {
+    if !hex.len().is_multiple_of(2) {
         anyhow::bail!("Hex string has odd length");
     }
     (0..hex.len())