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

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

crates/notcore/src/paths.rs

@@ -0,0 +1,42 @@
+use std::path::PathBuf;
+
+use crate::NotfilesError;
+
+pub fn expand_tilde(path: &str) -> Result<PathBuf, NotfilesError> {
+    if path == "~" {
+        return dirs::home_dir()
+            .ok_or_else(|| NotfilesError::Path("cannot determine home directory".into()));
+    }
+    if let Some(rest) = path.strip_prefix("~/") {
+        let home = dirs::home_dir()
+            .ok_or_else(|| NotfilesError::Path("cannot determine home directory".into()))?;
+        return Ok(home.join(rest));
+    }
+    Ok(PathBuf::from(path))
+}
+
+pub fn dotfiles_dir() -> Option<PathBuf> {
+    dirs::home_dir().map(|h| h.join("dotfiles"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_expand_tilde_home() {
+        let home = dirs::home_dir().unwrap();
+        assert_eq!(expand_tilde("~").unwrap(), home);
+    }
+
+    #[test]
+    fn test_expand_tilde_subpath() {
+        let home = dirs::home_dir().unwrap();
+        assert_eq!(expand_tilde("~/foo/bar").unwrap(), home.join("foo/bar"));
+    }
+
+    #[test]
+    fn test_expand_tilde_absolute() {
+        assert_eq!(expand_tilde("/usr/bin").unwrap(), PathBuf::from("/usr/bin"));
+    }
+}