[Issue]: Add public hooks for customizing subject labels, causer labels, and activity row display

[MrAdder]

Issue Type

Feature request

Summary

Expose safe extension points that let package consumers tailor how activity records are displayed.

Problem / Context

Some teams need the activity UI to reflect domain-specific naming or display conventions, but the current display behavior is relatively fixed.

Proposed Solution

Add public hooks or configuration-driven customization points for subject labels, causer labels, and activity row display formatting.

Acceptance Criteria

• Consumers can customize subject and causer display labels through supported hooks.
• Activity row display can be tailored without patching core classes.
• The extension points are documented and stable for 1.x.

Implementation Notes

Keep the API intentionally small and avoid exposing unstable internals. Favour explicit extension points over deep subclassing requirements.

Testing Notes

Add tests or fixtures that verify custom display hooks are applied consistently.

Additional Context

No response