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

tests/dockerignore_test.rs

@@ -84,9 +84,8 @@ fn pattern_matches(pattern: &str, path: &str) -> bool {
 fn is_excluded(patterns: &[String], path: &str) -> bool {
     let mut excluded = false;
     for pattern in patterns {
-        if pattern.starts_with('!') {
+        if let Some(negated) = pattern.strip_prefix('!') {
             // Negation pattern - re-include
-            let negated = &pattern[1..];
             if pattern_matches(negated, path) {
                 excluded = false;
             }