Rename "register agent" use case to "enroll agent"

[vikman90]

Description

We need to rename the "register agent" use case to "enroll agent" for consistency and clarity. This change affects the command-line option, internal class names, and documentation.

Tasks

• Rename the command-line option from --register-agent to --enroll-agent.
• Update the CLI help text to use "enroll" instead of "register".
• Rename the implementation file to agent_enrollment.cpp.
• Rename the namespace and class to AgentEnrollment.
• Update the server mock: files and comments.
• Update all relevant documentation to reflect this change.
• Ensure there are no remaining references to "register" or "registration" related to this use case.

Definition of Done

• The command-line option is --enroll-agent.
• The help text correctly refers to the use case as "enroll".
• The implementation file is named agent_enrollment.cpp.
• The namespace and class are named AgentEnrollment.
• The server mock has been updated to refer "enrollment".
• The documentation is updated accordingly.
• No references to "register" or "registration" remain in this context.