fix: honor cortex home for lock state

[Bortlesboat]

Summary

• route lock state path resolution through Cortex's existing home resolver
• keep lock state under the active Cortex home instead of hardcoded ~/.cortex
• add a regression test for CORTEX_HOME-backed lock file placement

Verification

• cargo test -p cortex-cli test_get_lock_file_path_uses_cortex_home -- --nocapture
• cargo test -p cortex-cli lock_cmd::tests -- --nocapture
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

[greptile-apps]

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