fix: honor cortex home for plugin storage

[Bortlesboat]

Summary

• Route plugin storage through the existing Cortex home resolver so CORTEX_HOME / CORTEX_CONFIG_DIR use <cortex-home>/plugins.
• Add regression coverage for get_plugins_dir() under a custom CORTEX_HOME.
• Fix the agent command test import so the focused cortex-cli lib test target compiles.

Related to PlatformNetwork/bounty-challenge#51564.

Verification

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

[greptile-apps]

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