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/channels/whatsapp.rs

@@ -294,7 +294,7 @@ mod tests {
         assert_eq!(msgs[0].sender, "+1234567890");
         assert_eq!(msgs[0].content, "Hello ZeroClaw!");
         assert_eq!(msgs[0].channel, "whatsapp");
-        assert_eq!(msgs[0].timestamp, 1699999999);
+        assert_eq!(msgs[0].timestamp, 1_699_999_999);
     }
 
     #[test]