feat(core): define error types with thiserror

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/core/src/error.rs

@@ -1 +1,164 @@
-// Task 3 will implement error types
+use std::path::PathBuf;
+
+#[derive(Debug, thiserror::Error)]
+pub enum CoreError {
+    #[error("I/O error: {context}: {source}")]
+    Io {
+        #[source]
+        source: std::io::Error,
+        context: String,
+    },
+
+    #[error("parse error at line {line}: {source}")]
+    Parse {
+        #[source]
+        source: serde_json::Error,
+        line: u64,
+    },
+
+    #[error("invalid search pattern '{pattern}': {reason}")]
+    Search { pattern: String, reason: String },
+
+    #[error("index error: {message}")]
+    Index { message: String },
+
+    #[error("config error: {source}")]
+    Config {
+        #[source]
+        source: toml::de::Error,
+    },
+
+    #[error("encoding error at line {line}: {bytes:?}")]
+    Encoding { line: u64, bytes: Vec<u8> },
+
+    #[error("file watch error: {0}")]
+    Watch(String),
+
+    #[error("file not found: {path:?}")]
+    FileNotFound { path: PathBuf },
+
+    #[error("{0}")]
+    Other(String),
+}
+
+pub type Result<T> = std::result::Result<T, CoreError>;
+
+impl From<std::io::Error> for CoreError {
+    fn from(err: std::io::Error) -> Self {
+        CoreError::Io {
+            source: err,
+            context: "I/O operation".into(),
+        }
+    }
+}
+
+impl From<serde_json::Error> for CoreError {
+    fn from(err: serde_json::Error) -> Self {
+        CoreError::Parse {
+            source: err,
+            line: 0,
+        }
+    }
+}
+
+impl From<toml::de::Error> for CoreError {
+    fn from(err: toml::de::Error) -> Self {
+        CoreError::Config { source: err }
+    }
+}
+
+impl From<notify::Error> for CoreError {
+    fn from(err: notify::Error) -> Self {
+        CoreError::Watch(err.to_string())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_io_error_display() {
+        let err = CoreError::Io {
+            source: std::io::Error::new(std::io::ErrorKind::NotFound, "file missing"),
+            context: "reading config".into(),
+        };
+        let msg = err.to_string();
+        assert!(
+            msg.contains("reading config"),
+            "display should contain context"
+        );
+        assert!(
+            msg.contains("file missing"),
+            "display should contain source message"
+        );
+    }
+
+    #[test]
+    fn test_parse_error_display() {
+        let json_err: serde_json::Error =
+            serde_json::from_str::<serde_json::Value>("{bad}").unwrap_err();
+        let err = CoreError::Parse {
+            source: json_err,
+            line: 42,
+        };
+        let msg = err.to_string();
+        assert!(msg.contains("42"), "display should contain line number");
+    }
+
+    #[test]
+    fn test_search_error_display() {
+        let err = CoreError::Search {
+            pattern: "[invalid".into(),
+            reason: "unclosed bracket".into(),
+        };
+        let msg = err.to_string();
+        assert!(msg.contains("[invalid"), "display should contain pattern");
+        assert!(
+            msg.contains("unclosed bracket"),
+            "display should contain reason"
+        );
+    }
+
+    #[test]
+    fn test_from_io_error() {
+        let io_err = std::io::Error::new(std::io::ErrorKind::PermissionDenied, "access denied");
+        let core_err: CoreError = io_err.into();
+        match core_err {
+            CoreError::Io { context, .. } => assert_eq!(context, "I/O operation"),
+            other => panic!("expected Io variant, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn test_from_notify_error() {
+        let notify_err = notify::Error::watch_not_found();
+        let core_err: CoreError = notify_err.into();
+        match core_err {
+            CoreError::Watch(msg) => assert!(!msg.is_empty()),
+            other => panic!("expected Watch variant, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn test_result_type_alias() {
+        let ok: Result<i32> = Ok(42);
+        let err: Result<i32> = Err(CoreError::Other("something went wrong".into()));
+        assert!(ok.is_ok());
+        assert!(err.is_err());
+    }
+
+    #[test]
+    fn test_error_source_chain() {
+        let io_err = std::io::Error::new(std::io::ErrorKind::UnexpectedEof, "unexpected eof");
+        let core_err = CoreError::Io {
+            source: io_err,
+            context: "test".into(),
+        };
+        let source = std::error::Error::source(&core_err);
+        assert!(
+            source.is_some(),
+            "source() should return the inner io::Error"
+        );
+    }
+}