AI: Group custom agents by discovery source in the Agent picker

## Feature request

Group custom agents in the Agent picker by the location or source where VS Code discovered them.

## Problem

When many custom agents are available, the picker shows one long, flat list. It is difficult to tell:

- whether an agent comes from an organization, a plugin, an extension, the repository, or the user profile;
- which organization owns an organization-level agent;
- why an agent is available in the current context;
- which agent to choose when names are similar.

The list becomes difficult to scan and manage as more agents are installed or discovered.

## Proposed UI

Use collapsible groups based on the discovery source, with a second level for organization names:

```text
Organization agents
org1
org1's custom agent 1
org1's custom agent 2
org2
org2's custom agent 1

Agents from installed plugins
plugin agent 1

Agents from extensions
extension agent 1

Repository agents
repository custom agent 1

User agents
user custom agent 1

Other locations
agent from a configured or unknown location
```

Suggested behaviour:

- Show only groups that contain agents.
- Allow each group to expand and collapse.
- Keep the current agent search, filtering, keyboard navigation, and selection behaviour.
- Keep the current discovery, de-duplication, precedence, and execution behaviour unchanged.
- Show the organization, extension, plugin, or file location as source information where it is available, for example in a group label or tooltip.
- Preserve the current ordering within each group unless there is a strong reason to change it.
- Use **Other locations** as a catch-all for configured locations that do not match a known source category.
- If an agent is discovered from more than one source, retain the existing resolution behaviour and show the relevant source information where possible.

## Benefits

This would make the picker easier to scan and would help users understand the scope and ownership of each agent. It would also scale better for users who have agents from several organizations, extensions, plugins, repositories, and user-level locations.

This request is related to #332543, but focuses specifically on a source-based hierarchy for custom agents in the Agent picker rather than the general display of all customization types.