Fix cache command path mismatch

[Bortlesboat]

Summary

• Make cache subcommands use the same Cortex-home cache directory reported by debug paths.
• Add a binary integration regression that compares cache show --json with debug paths --json.

Tests

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

Note: I avoided the full cortex-cli lib-test route because current main has an unrelated private import compile error in agent_cmd/tests.rs.

[greptile-apps]

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