feat(notcore): add shared types, config, paths, error crate

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/notcore/src/lib.rs

@@ -0,0 +1,9 @@
+pub mod config;
+pub mod error;
+pub mod paths;
+pub mod types;
+
+pub use config::{Config, Defaults, Method, PackageConfig};
+pub use error::NotfilesError;
+pub use paths::{dotfiles_dir, expand_tilde};
+pub use types::{HookPhase, HookSpec, PackageSpec, Report, Step, StepStatus};