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/tools/browser.rs

@@ -365,8 +365,8 @@ impl BrowserTool {
     }
 }
 
-#[async_trait]
 #[allow(clippy::too_many_lines)]
+#[async_trait]
 impl Tool for BrowserTool {
     fn name(&self) -> &str {
         "browser"
@@ -750,7 +750,7 @@ mod tests {
         let domains = vec![
             "  Example.COM  ".into(),
             "docs.example.com".into(),
-            "".into(),
+            String::new(),
         ];
         let normalized = normalize_domains(domains);
         assert_eq!(normalized, vec!["example.com", "docs.example.com"]);