[feature]: API endpoint to list workspaces accessible to the current token

Is your feature request related to a problem?

There is no way for an API token holder to discover their workspace slug programmatically. GET /api/v1/users/me/ works and returns the user profile, but there is no companion endpoint listing workspaces:

GET /api/v1/workspaces/            → 404
GET /api/v1/users/me/workspaces/   → 404

Every other v1 endpoint requires the workspace slug in the path, so integrations cannot bootstrap themselves from a token alone. In practice the user has to read the slug out of the browser address bar and paste it into the script or config.

Describe the solution you'd like

A read only endpoint such as GET /api/v1/users/me/workspaces/ returning the id, name, and slug of each workspace the token can access. This mirrors what the web app already knows and would make first run setup for scripts, MCP servers, and other integrations self service.

Describe alternatives you've considered

Asking the user for the slug manually (current workaround), or parsing it from a pasted URL. Both work but add friction to every integration's onboarding.

Environment

Plane Cloud, public REST API v1, personal API token.

Start by locating the existing GET /api/v1/users/me/ entry point and the workspace-access rules used by v1 endpoints. Confirm the response fields and access semantics for the proposed workspace-list endpoint, then verify that it is read-only and returns every workspace accessible to the current token.