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/mod.rs

@@ -0,0 +1,42 @@
+//! Agent Skills support for G3.
+//!
+//! Implements the Agent Skills specification (https://agentskills.io)
+//! for discovering and using portable skill packages.
+//!
+//! # Overview
+//!
+//! Skills are packages of instructions that give the agent new capabilities.
+//! Each skill is a directory containing a `SKILL.md` file with:
+//! - YAML frontmatter (name, description, metadata)
+//! - Markdown body with detailed instructions
+//!
+//! # Directory Structure
+//!
+//! ```text
+//! skill-name/
+//! ├── SKILL.md          # Required: instructions + metadata
+//! ├── scripts/          # Optional: executable code
+//! ├── references/       # Optional: additional documentation  
+//! └── assets/           # Optional: templates, data files
+//! ```
+//!
+//! # Discovery
+//!
+//! Skills are discovered from:
+//! 1. Global: `~/.g3/skills/` (lowest priority)
+//! 2. Extra paths from config (medium priority)
+//! 3. Workspace: `.g3/skills/` (highest priority, overrides others)
+//!
+//! # Usage
+//!
+//! 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.
+
+mod parser;
+mod discovery;
+mod prompt;
+
+pub use parser::Skill;
+pub use discovery::discover_skills;
+pub use prompt::generate_skills_prompt;