Split CLI LLM provider config

[haasonsaas]

Summary

• move CLI LLM provider/model/env/header wiring into crates/browser-use-cli/src/llm_config.rs
• keep main.rs focused on command parsing and execution while preserving the existing clap/MCP provider surface
• reduce crates/browser-use-cli/src/main.rs from 1,217 to 933 lines

Test Plan

• cargo check -p browser-use-cli
• cargo test -p browser-use-cli
• cargo fmt --check
• git diff --check