feat: integrate open-skills library and cleanup clippy warnings

- Add open-skills auto-clone/pull/sync support in skills loader
  - Clone https://github.com/besoeasy/open-skills to ~/open-skills
  - Weekly sync via .zeroclaw-open-skills-sync marker
  - Env controls: ZEROCLAW_OPEN_SKILLS_ENABLED, ZEROCLAW_OPEN_SKILLS_DIR
  - Load open-skills markdown files before workspace skills
  - Track Skill.location for accurate prompt rendering

- Update system prompt to render skill.location with fallback
  - Use actual file path when available
  - Maintain backward compatibility with workspace SKILL.md path

- Fix clippy warnings across tests and supporting files
  - Readable timestamp literals
  - Remove underscore bindings in tests
  - Use struct update syntax for Config::default() patterns
  - Fix module inception, duplicate attributes, manual strip
  - Clean raw string hashes and empty string construction

Resolves: #77

src/security/secrets.rs

@@ -242,7 +242,7 @@ fn hex_encode(data: &[u8]) -> String {
 /// Hex-decode a hex string to bytes.
 #[allow(clippy::manual_is_multiple_of)]
 fn hex_decode(hex: &str) -> Result<Vec<u8>> {
-    if !hex.len().is_multiple_of(2) {
+    if (hex.len() & 1) != 0 {
         anyhow::bail!("Hex string has odd length");
     }
     (0..hex.len())