Fix: update --add-dir help text to use plural DIRS placeholder

[watcharaponthod-code]

This PR addresses the inconsistency identified in PlatformNetwork/bounty-challenge#53507.

Changes

• Updated the value_name of the --add-dir argument from DIR to DIRS in src/cortex-cli/src/cli/args.rs.
• This matches the plural context of the help description ("Additional directories") and clarifies that multiple directories can be provided (via repeated flags), following standard CLI conventions.

Verification

• Verified the source code change in args.rs.
• Ensures that cortex --help will now display --add-dir <DIRS> instead of the confusing singular form.

[greptile-apps]

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