Allow Guest+ to execute custom agents in foreground (Agentic Chat)
## Summary
As part of the DAP permission https://gitlab.com/groups/gitlab-org/-/epics/19743+, we need to loosen permissions for custom agents executing in the foreground (Agentic Chat) from Developer+ to Guest+.
## Background
Currently, custom agents accessible in Agentic Chat require Developer+ permissions. However, there is no technical reason preventing Guest+ access for foreground execution of custom agents.
By allowing Guest+ to execute custom agents in the foreground, we can create a unified "Execute foreground" permission that covers:
- Agentic Chat
* Foundational agents (foreground)
* Custom agents (foreground)
- Foreground flows: Software Development Flow in the IDE.
## Implementation
Update the relevant policy files to allow Guest+ access for custom agent execution in the foreground (Agentic Chat context).
## Constraints
Background execution (requiring CI Pipeline) will continue to require Developer+ permissions.
## Related
- Spike issue: #582055
- Related MR: (will be linked once foreground flows MR is created)
issue