feat(init): scaffold cargo workspace and CI

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

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

crates/core/src/error.rs

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+// Task 3 will implement error types

crates/core/src/lib.rs

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+pub mod error;
+pub mod types;
+// pub mod io; — Task 4
+// pub mod search; — Task 4
+// pub mod parser; — Task 4
+// pub mod watcher; — Task 4
+// pub mod filter; — Task 4
+// pub mod bookmark; — Task 4
+// pub mod config; — Task 4
+// pub mod session; — Task 4

crates/core/src/types.rs

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+// Task 2 will implement types