Reject plugin path arguments for installed operations

[GreyforgeLabs]

Summary

• Validate installed plugin names as a single path component before remove/enable/disable/show build paths under the plugins directory.
• Reject absolute paths, nested paths, traversal components, and Windows-style separators instead of passing them through PathBuf::join.
• Add a binary integration regression proving cortex plugin remove -y <absolute-path> leaves the outside directory intact.

Fixes PlatformNetwork/bounty-challenge#49473.

Validation

• cargo test -p cortex-cli --test plugin_remove_path --no-default-features --features cortex-tui -- --nocapture
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo fmt --package cortex-cli --check
• git diff --check

[greptile-apps]

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