Make research skill self-contained without external scripts

- Rewrite SKILL.md with inline instructions to spawn g3 --agent scout directly
- Extend read_file to handle embedded skill paths (<embedded:name>/SKILL.md)
- Remove scripts field from EmbeddedSkill struct (no longer needed)
- Delete extraction.rs module (was only for script extraction)
- Delete g3-research bash script
- Remove obsolete Async Research Tool section from workspace memory

Skills are now fully portable - they work when g3 is installed as a
binary without access to source files. Agents can read embedded skill
content via read_file with the special <embedded:...> path syntax.

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

@@ -39,7 +39,6 @@ mod parser;
 mod discovery;
 mod prompt;
 mod embedded;
-pub mod extraction;
 
 pub use parser::Skill;
 pub use discovery::discover_skills;