Respect CORTEX_HOME for uninstall backups

[GreyforgeLabs]

Summary

• Use the resolved Cortex home directory for uninstall backup roots, so CORTEX_HOME places backups under that relocated state tree.
• Collect uninstall candidates from the same resolved Cortex home instead of hardcoding $HOME/.cortex.
• Avoid recursive self-copying when backups live under Cortex home, and keep dry-run backup previews non-destructive.
• Add a CLI regression for uninstall --backup --dry-run with CORTEX_HOME set.

Fixes PlatformNetwork/bounty-challenge#52799.

Validation

• cargo test -p cortex-cli --test uninstall_backup --no-default-features --features cortex-tui -- --nocapture
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo fmt --package cortex-cli --check
• git diff --check

[greptile-apps]

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