Feature request: Make @ mentions extensible for integrations

What variant of Codex are you using?

CLI, Codex App, and IDE Extension

What feature would you like to see?

Please make the @ mention system extensible so integrations can contribute their own mentionable resources to Codex.

Codex already provides @ mentions for native resources. Integrations should have a supported way to make external resources discoverable and selectable through the same experience, including resources that may change while Codex is running.

At a product level:

• Integration-provided resources can appear in @ suggestions alongside native resources.
• Users can search for and select those resources using the standard mention experience.
• The selected resource keeps its identity and can be handled by the contributing integration when the prompt is submitted.
• Integrations can update their available resources without requiring changes to Codex itself.
• An unavailable integration does not break native @ behavior.

This request is intentionally implementation-neutral. It is not tied to a particular integration, resource type, transport, protocol, or Codex surface. The goal is a supported extension point so @ mentions can be extended without forking Codex or intercepting terminal input.

Additional information

Examples of mentionable resources could include repositories, documents, issues, services, people, agents, environments, or other objects from external tools. The exact extension mechanism should follow Codex's architecture.

The issue names no files, tests, or entry points; begin by locating Codex's existing @-mention implementation across the CLI, App, and IDE Extension. Define the extension boundary from the stated requirements, including dynamic resources, preserved identity, integration handling, and graceful unavailability. Done means integrations can add searchable resources without modifying Codex or breaking native mentions.