[Issue]: Add clearer extension points for custom tabs, widgets, filters, and alert registration

[MrAdder]

Issue Type

Feature request

Summary

Make advanced customization easier by formalizing supported extension points around resource behavior and alert registration.

Problem / Context

Consumers may want to add project-specific tabs, widgets, filters, or alert rules, but the supported customization path is not yet explicit.

Proposed Solution

Introduce clearer extension points for custom resource tabs, widgets, filters, and alert rule registration in a way that stays stable through 1.x.

Acceptance Criteria

• Consumers can extend tabs, widgets, filters, or alert registration through supported hooks.
• The extension path is documented.
• The new extension surface is stable and does not require patching internals.

Implementation Notes

Review current resource and alert registration flow to identify the smallest stable surface worth formalizing.

Testing Notes

Add tests that exercise custom registration or extension behavior through the supported API.

Additional Context

This should make the package easier to extend without committing to more surface area than needed.