fix: include agent config in uninstall purge

[Bortlesboat]

Summary

• Include agents.toml and aliases.toml in the ~/.cortex config files collected by cortex uninstall --purge.
• Add temp-home coverage for the uninstall collector so future config files do not silently survive purge.
• Fix a test-only import for read_file_with_encoding so focused cortex-cli unit tests compile on current main.

Related to PlatformNetwork/bounty-challenge#47779.

Verification

• cargo test -p cortex-cli test_collect_cortex_home_items_includes_agent_config_files -- --nocapture
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

[greptile-apps]

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