## Why
The app directory is a discovery catalog and can include apps that are not
callable in the active thread. Using it for `$` suggestions can therefore
offer mentions that cannot be used.
## What changed
- Load mention candidates from `app/installed`, enrich callable apps through
bounded `app/read` batches, and retain compatibility with app servers that
do not support `app/installed`.
- Keep the mention catalog separate from the `/apps` directory cache and scope
results to the current thread, workspace, and account.
- Refresh mentions after app notifications, enablement changes, configuration
updates, and apps MCP readiness, while immediately removing disabled apps.
## Testing
Add coverage for batching and fallback behavior, stale scoped results,
notification refreshes, enablement changes, MCP startup races, and filtering
non-callable apps from mentions.
GitOrigin-RevId: 4d9e20794aed30cc97120170792fb555b86fe1e7
Charlie Marsh
fe5889928c