Add a central instruction/context file for shared room guidance

### User Problem
I want to define some instructions and context once, instead of teaching the same things to every room individually. Repeating shared guidance across rooms is inefficient and easy to forget.

### Feature Request
Add a central instruction/context file, similar to Claude.md, that can provide shared guidance across rooms.

**Example:**

- User defines global project instructions in one central file
- Multiple rooms can use the same shared instructions
- User does not need to repeat the same context in every room
- Room-specific instructions can still override or extend the central context

Start by locating the room instruction and context-loading entry points in the TypeScript codebase, then trace how room-specific guidance is stored and applied. Define the central instruction/context file and verify that multiple rooms inherit it while room-specific instructions can override or extend it.

Written by the indexing model from the issue text.