:recycle: refactor: unify project naming to SkCC across all docs, comments, and configs

- README.md: SkillCompiler/NSC → SkCC, update citation to arXiv paper
- PROGRESS.md: Nexa Skill Compiler (NSC) → SkCC
- Cargo.toml: update description and repository URL
- docs/: all 15+ files unified to SkCC
- Rust source comments: lib.rs, main.rs, skill_ir.rs, etc.
- npm package: README, package.json repository URL
- VS Code extension: displayName, commands, status bar, diagnostics
- Preserved: crate names, npm package name, CLI binary 'nsc', VS Code command IDs