refactor(core): split agent public types

[haasonsaas]

Summary

• extract the agent task, checkpoint, run error, and callback type contracts into agent/types.rs
• keep agent.rs re-exporting the same public names for the crate root
• reduce agent.rs from 1,473 lines on main to 1,303 lines in this branch

Validation

• cargo check -p browser-use-core
• cargo test -p browser-use-core checkpoint --lib
• cargo test -p browser-use-core callback --lib
• cargo test -p browser-use-core --lib
• cargo fmt --check
• git diff --check