Feature Request: Install and Assign Pi Extensions to Rooms

## Context

Pi extensions should not only be installable, but also usable within Exxperts and assignable to individual rooms. This would allow users to enable specific functionality where it is needed while keeping room contexts focused and isolated.

## Proposed Solution
Add support for managing Pi extensions with the following capabilities:
- Install extensions from a repository URL, for example from GitHub.
- List and manage installed extensions.
- Assign extensions to one or more rooms.
- Load assigned extensions automatically when working in a room.
- Prevent extensions assigned to one room from being available in unrelated rooms.

## Example
A user should be able to install and use pi-goal (https://github.com/Michaelliv/pi-goal) in a specific room:
1. Install the extension from its repository.
2. Open the desired room.
3. Assign pi-goal to the room.
4. Use the extension's functionality while working in that room.

## Acceptance Criteria
- Users can install a Pi extension by providing a repository URL.
- Users can list and manage installed extensions.
- An installed extension can be assigned to one or more rooms.
- Extensions assigned to a room are automatically available when working in that room.
- Extensions are not available in rooms to which they have not been assigned.
- Users can remove an extension from a room without uninstalling it globally.
- The installation, assignment, and usage workflow is documented with an example using pi-goal.

The issue does not name files, tests, or entry points. Start by tracing the existing extension installation and room-management flows, then determine where installed extensions and room assignments are stored and loaded. Done means the listed installation, assignment, isolation, removal, and pi-goal documentation criteria are covered and verified by tests.

Written by the indexing model from the issue text.