feat: Create a new HookEvent for Multiagent

[JackYPCOnline]

Description

We need a new hook event class that does not have agent as parameter for Multiagent.
So this PR creates a new hook event base class. The old single agent hook extend this hook base class, so it remains the same.

Is this meant to replace HookEvent?

We have to introduce a new base event that doesn't have an Agent property; we can't update the old event, because folks could be using it already.

If we were doing this from the get-go to support Multi-Agent, we'd most likely just be adding agent: Agent to each non-multi-agent hook instead of having it in the base class, but hooks didn't account for multi-agent

Related Issues

Documentation PR

Type of Change

Bug fix
New feature
Breaking change
Documentation update
Other (please describe):

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• [ x] I ran hatch run prepare

Checklist

• [ x] I have read the CONTRIBUTING document
• [ x] I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• [ x] My changes generate no new warnings
• [x ] Any dependent changes have been merged and published

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