Prune unused dependencies and simplify performance monitor initialization

[vinhnx]

Summary

• remove unused direct dependencies from the CLI crate and move test-only crates into dev-dependencies
• drop unused crates from the vtcode-core library manifest and consolidate on once_cell for lazy initialization
• simplify the performance monitor report generation and replace lazy_static with once_cell::Lazy
• refresh Cargo.lock to reflect the trimmed dependency graph

Testing

• cargo fmt
• cargo check
• cargo test

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https://chatgpt.com/codex/tasks/task_e_68e480569e4083238eda255ae8aad55c