Support combining user-wide and project-specific Executor configuration #1374

I want to configure some Executor integrations once at a user/system level, while still allowing individual projects to have their own Executor configuration.

This is important to me because currently I scope each executor to each project configuration, allowing each project to define the correct integrations. However, there are some global integrations that I'd like to do.

Example use case:

• I want Slack and Linear available across all of my work projects.
• I do not want to re-add Slack and Linear separately in every project (what I do currently)
• I still want each project to be able to define its own project-specific Executor config
• Ideally, when I run Executor inside a project, it would use both my shared user-level integrations and that project’s local configuration

I think something not far from how OpenCode merges config could be an acceptable solution. In an ideal world, I can actually use a script (plugin) to add integrations to my executor on-demand, e.g., if running under dir1, enable N integrations, if running under dir2, run Z integrations.