fix: include purge data directories

[Bortlesboat]

Summary

• Include ~/.cortex/delegates and ~/.cortex/snapshots in the data directories collected by cortex uninstall --purge.
• Add temp-home coverage for the uninstall collector so these generated data directories are listed for 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#52677.

Verification

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

[greptile-apps]

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