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-config/src/lib.rs

@@ -13,6 +13,8 @@ pub struct Config {
     pub computer_control: ComputerControlConfig,
     #[serde(default)]
     pub webdriver: WebDriverConfig,
+    #[serde(default)]
+    pub skills: SkillsConfig,
 }
 
 /// Provider configuration with named configs per provider type
@@ -193,6 +195,26 @@ pub struct WebDriverConfig {
     pub browser: WebDriverBrowser,
 }
 
+/// Skills configuration
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SkillsConfig {
+    /// Whether skills are enabled (default: true)
+    #[serde(default = "default_true")]
+    pub enabled: bool,
+    /// Additional paths to search for skills (beyond ~/.g3/skills and .g3/skills)
+    #[serde(default)]
+    pub extra_paths: Vec<String>,
+}
+
+impl Default for SkillsConfig {
+    fn default() -> Self {
+        Self {
+            enabled: true,
+            extra_paths: Vec::new(),
+        }
+    }
+}
+
 impl Default for AgentConfig {
     fn default() -> Self {
         Self {
@@ -257,6 +279,7 @@ impl Default for Config {
             },
             computer_control: ComputerControlConfig::default(),
             webdriver: WebDriverConfig::default(),
+            skills: SkillsConfig::default(),
         }
     }
 }