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/cron/scheduler.rs

@@ -186,9 +186,11 @@ mod tests {
     use tempfile::TempDir;
 
     fn test_config(tmp: &TempDir) -> Config {
-        let mut config = Config::default();
-        config.workspace_dir = tmp.path().join("workspace");
-        config.config_path = tmp.path().join("config.toml");
+        let config = Config {
+            workspace_dir: tmp.path().join("workspace"),
+            config_path: tmp.path().join("config.toml"),
+            ..Config::default()
+        };
         std::fs::create_dir_all(&config.workspace_dir).unwrap();
         config
     }