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/lib.rs

@@ -23,6 +23,7 @@ pub mod tools;
 pub mod ui_writer;
 pub mod utils;
 pub mod webdriver_session;
+pub mod skills;
 
 pub use feedback_extraction::{
     extract_coach_feedback, ExtractedFeedback, FeedbackExtractionConfig, FeedbackSource,
@@ -42,7 +43,13 @@ pub use context_window::{ContextWindow, ThinResult, ThinScope};
 pub use pending_research::{PendingResearchManager, ResearchCompletionNotification, ResearchStatus};
 
 // Export agent prompt generation for CLI use
-pub use prompts::get_agent_system_prompt;
+pub use prompts::{
+    get_agent_system_prompt, get_agent_system_prompt_with_skills,
+    get_system_prompt_for_native_with_skills, get_system_prompt_for_non_native_with_skills,
+};
+
+// Export skills module
+pub use skills::{Skill, discover_skills, generate_skills_prompt};
 
 #[cfg(test)]
 mod task_result_comprehensive_tests;