feat: Externalize research tool as embedded skill

Replaces the built-in research/research_status tools with a portable
skill-based approach:

- Add embedded skills infrastructure (skills compiled into binary)
- Add repo-local skills/ directory support (highest priority)
- Create research skill with SKILL.md and g3-research shell script
- Script extraction to .g3/bin/ with version tracking
- Filesystem-based handoff via .g3/research/<id>/status.json
- Remove PendingResearchManager and all research tool code
- Update system prompt to reference skill instead of tool

Benefits:
- No special tool infrastructure needed (just shell + read_file)
- Context-efficient (reports stay on disk until needed)
- Crash-resilient (state persisted to filesystem)
- Portable (skill can be overridden per-workspace)

Breaking change: research tool calls now return a deprecation message
pointing to the research skill.

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

@@ -22,10 +22,12 @@
 //!
 //! # 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)
+//! Skills are discovered from (highest to lowest priority):
+//! 1. Repo: `skills/` at repo root (checked into git, overrides all)
+//! 2. Workspace: `.g3/skills/` (local customizations)
+//! 3. Extra paths from config
+//! 4. Global: `~/.g3/skills/`
+//! 5. Embedded: compiled into binary (always available)
 //!
 //! # Usage
 //!
@@ -36,7 +38,10 @@
 mod parser;
 mod discovery;
 mod prompt;
+mod embedded;
+pub mod extraction;
 
 pub use parser::Skill;
 pub use discovery::discover_skills;
 pub use prompt::generate_skills_prompt;
+pub use embedded::{get_embedded_skills, get_embedded_skill, EmbeddedSkill};