fix: return non-zero on import-dir failures

[cotishq]

Description

• Converts import-dir command execution to Cobra RunE so validation/config/import errors are returned instead of silently exiting successfully.
• Returns a non-zero exit status when one or more artifact imports fail, including partial and total import failure cases.
• Updates the failure summary message to make failed imports explicit in CLI/CI logs.
• Adds tests for import result failure handling and success/failure summary output.

Related issue(s)

Fixes #379

Proof of Work