fix: honor cortex home for dag storage

[Bortlesboat]

Summary

• Route DAG storage through the existing find_cortex_home() resolver so CORTEX_CONFIG_DIR and CORTEX_HOME store DAGs under <cortex-home>/dags.
• Add regression coverage for both CORTEX_HOME and CORTEX_CONFIG_DIR precedence.
• Fix the agent command test import so the focused cortex-cli lib test target compiles.

Related to PlatformNetwork/bounty-challenge#52498.

Verification

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

[greptile-apps]

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