feat: implement Agent Skills specification support

Implements the Agent Skills specification (https://agentskills.io) for
portable skill packages that give the agent new capabilities.

Changes:
- Add skills module with SKILL.md parser (YAML frontmatter + markdown body)
- Implement skill discovery from ~/.g3/skills/, config extra_paths, and .g3/skills/
- Generate <available_skills> XML for system prompt injection
- Add SkillsConfig to g3-config with enabled flag and extra_paths
- Wire skills discovery into CLI startup
- Add 29 unit tests for parser, discovery, and prompt generation
- Update README with Agent Skills documentation

Skill locations (priority order):
1. ~/.g3/skills/ (global)
2. Config extra_paths
3. .g3/skills/ (workspace, highest priority)

At startup, g3 scans skill directories and injects a summary into the
system prompt. When the agent needs a skill, it reads the full SKILL.md
using the read_file tool.

crates/g3-core/src/skills/prompt.rs

@@ -0,0 +1,140 @@
+//! Generate XML prompt section for available skills.
+//!
+//! Creates the `<available_skills>` XML block that gets injected into
+//! the system prompt to inform the agent about available skills.
+
+use super::parser::Skill;
+
+/// Generate the XML section for available skills.
+///
+/// Returns an empty string if no skills are available.
+/// The XML format follows the Agent Skills specification.
+pub fn generate_skills_prompt(skills: &[Skill]) -> String {
+    if skills.is_empty() {
+        return String::new();
+    }
+    
+    let mut xml = String::new();
+    xml.push_str("# Available Skills\n\n");
+    xml.push_str("You have access to the following skills. When a task matches a skill's description, \
+                  read the full skill file using `read_file` to get detailed instructions.\n\n");
+    xml.push_str("<available_skills>\n");
+    
+    for skill in skills {
+        xml.push_str("  <skill>\n");
+        xml.push_str(&format!("    <name>{}</name>\n", escape_xml(&skill.name)));
+        xml.push_str(&format!("    <description>{}</description>\n", escape_xml(&skill.description)));
+        xml.push_str(&format!("    <location>{}</location>\n", escape_xml(&skill.path)));
+        
+        // Include compatibility info if present
+        if let Some(ref compat) = skill.compatibility {
+            xml.push_str(&format!("    <compatibility>{}</compatibility>\n", escape_xml(compat)));
+        }
+        
+        xml.push_str("  </skill>\n");
+    }
+    
+    xml.push_str("</available_skills>\n");
+    xml
+}
+
+/// Escape special XML characters.
+fn escape_xml(s: &str) -> String {
+    s.replace('&', "&amp;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+        .replace('"', "&quot;")
+        .replace('\'', "&apos;")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    
+    fn make_skill(name: &str, description: &str, path: &str) -> Skill {
+        Skill {
+            name: name.to_string(),
+            description: description.to_string(),
+            license: None,
+            compatibility: None,
+            metadata: None,
+            allowed_tools: None,
+            body: String::new(),
+            path: path.to_string(),
+        }
+    }
+    
+    #[test]
+    fn test_empty_skills() {
+        let result = generate_skills_prompt(&[]);
+        assert!(result.is_empty());
+    }
+    
+    #[test]
+    fn test_single_skill() {
+        let skills = vec![
+            make_skill("pdf-processing", "Extract text from PDFs", "/home/user/.g3/skills/pdf-processing/SKILL.md"),
+        ];
+        
+        let result = generate_skills_prompt(&skills);
+        
+        assert!(result.contains("<available_skills>"));
+        assert!(result.contains("</available_skills>"));
+        assert!(result.contains("<name>pdf-processing</name>"));
+        assert!(result.contains("<description>Extract text from PDFs</description>"));
+        assert!(result.contains("<location>/home/user/.g3/skills/pdf-processing/SKILL.md</location>"));
+    }
+    
+    #[test]
+    fn test_multiple_skills() {
+        let skills = vec![
+            make_skill("skill-a", "First skill", "/path/a/SKILL.md"),
+            make_skill("skill-b", "Second skill", "/path/b/SKILL.md"),
+        ];
+        
+        let result = generate_skills_prompt(&skills);
+        
+        assert!(result.contains("<name>skill-a</name>"));
+        assert!(result.contains("<name>skill-b</name>"));
+        // Should have exactly 2 skill blocks
+        assert_eq!(result.matches("<skill>").count(), 2);
+        assert_eq!(result.matches("</skill>").count(), 2);
+    }
+    
+    #[test]
+    fn test_xml_escaping() {
+        let skills = vec![
+            make_skill("test-skill", "Handle <special> & \"characters\"", "/path/SKILL.md"),
+        ];
+        
+        let result = generate_skills_prompt(&skills);
+        
+        assert!(result.contains("&lt;special&gt;"));
+        assert!(result.contains("&amp;"));
+        assert!(result.contains("&quot;characters&quot;"));
+        // Should not contain unescaped special chars in description
+        assert!(!result.contains("<special>"));
+    }
+    
+    #[test]
+    fn test_with_compatibility() {
+        let mut skill = make_skill("docker-skill", "Docker operations", "/path/SKILL.md");
+        skill.compatibility = Some("Requires Docker 20.0+".to_string());
+        
+        let result = generate_skills_prompt(&[skill]);
+        
+        assert!(result.contains("<compatibility>Requires Docker 20.0+</compatibility>"));
+    }
+    
+    #[test]
+    fn test_header_text() {
+        let skills = vec![
+            make_skill("test", "Test skill", "/path/SKILL.md"),
+        ];
+        
+        let result = generate_skills_prompt(&skills);
+        
+        assert!(result.contains("# Available Skills"));
+        assert!(result.contains("read the full skill file using `read_file`"));
+    }
+}