Feature request: Group custom agents by discovery source

## Feature request

Improve the custom agents view and agent picker by grouping agents according to where they were discovered.

## Problem

When custom agents from multiple sources appear in one flat list, it is difficult to find agents from a specific organisation, understand their scope and ownership, and manage a large number of agents.

## Proposed UI

Display custom agents in collapsible groups with organisation-level agents nested by organisation:

- Organisation-level agents
- org1
- org1's custom agent 1
- org1's custom agent 2
- org2
- Agents from installed plugins
- Agents from extensions
- Repository-level custom agents
- User-level custom agents
- Other locations

## Expected behaviour

- Group agents by their discovery source.
- Nest organisation-level agents under the relevant organisation.
- Allow each group to be expanded or collapsed.
- Keep search and filtering available across all groups.
- Show the source location or scope in the agent details or tooltip.
- Disambiguate agents with the same name.
- Place agents with an unknown or unsupported source under `Other locations`.
- Preserve existing actions for selecting and running an agent.
- Support keyboard navigation and screen readers.

## Benefits

This would improve discoverability and make the scope, ownership, and origin of each custom agent clear. It would also make the agent picker easier to use for users who work with several organisations, repositories, extensions, and plugins.

## Non-goals

This request does not propose changing how custom agents are discovered, loaded, or executed. It proposes a clearer presentation and organisation of the existing agents only.

## Acceptance criteria

- Custom agents are grouped by discovery source.
- Organisation-level agents are grouped by organisation.
- Groups can be expanded and collapsed.
- Search returns agents from every group.
- Duplicate agent names are clearly distinguished.
- The UI remains accessible and existing agent actions continue to work.