docs(readme): separate Copilot setup command

[zzz100868]

Summary

• Separate the Claude Code and GitHub Copilot setup commands in Quick Start.
• Show the required --copilot flag for GitHub Copilot.

Fixes #2244.

Test plan

• Verified the Quick Start commands match the Supported AI Tools table.
• cargo fmt --all && cargo clippy --all-targets && cargo test (Rust toolchain is not available locally.)

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