Fix plugin new cd hint for custom output dirs

[OnlyYu1996]

Summary

• print the actual created plugin directory in the plugin new next-step cd hint
• keep default output behavior unchanged while making custom --output directories copy-pasteable
• fix a stale test import so cortex-cli tests compile on current main

Fixes PlatformNetwork/bounty-challenge#53131

Verification

• cargo fmt --package cortex-cli --check
• cargo test --package cortex-cli plugin_cmd::tests::test_plugin_new_cd_target_uses_created_directory -- --nocapture
• cargo test --package cortex-cli plugin_cmd::tests -- --nocapture
• cargo check --package cortex-cli --lib
• git diff --check
• cargo run --package cortex-cli --bin Cortex -- plugin new sample-output-plugin --output /tmp/.../out-plugins --author Codex

[greptile-apps]

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