fix: honor cortex config dir for whoami

[Bortlesboat]

Summary

• Route cortex whoami stored-auth lookup through Cortex's existing home resolver so CORTEX_CONFIG_DIR / CORTEX_HOME are honored instead of always reading from ~/.cortex.
• Add regression coverage for the whoami Cortex home resolver under a custom CORTEX_CONFIG_DIR.
• Fix the agent command test import so the focused cortex-cli lib test target compiles.

Related to PlatformNetwork/bounty-challenge#51576.

Verification

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

[greptile-apps]

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