feat(pi): add agent integration

[YHM404]

Summary

• add a thin pi extension that overrides pi's bash tool and delegates to rtk rewrite
• add rtk init -g --agent pi install, uninstall, and config-status support
• document pi in the supported tools list and installation examples

Test plan

• cargo fmt --all --check && cargo clippy --all-targets && cargo test
• Manual testing: installed the extension in pi, ran /reload, and verified git status was rewritten to RTK output

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