Shared vars - roomd

Shared vars hold small facts a room needs: a port, a staging URL, a feature flag, a counter. Every tool takes room_id.

Tool	Arguments	Returns
`set_shared_var`	`roomId`, `key`, `value` (string), `agentId`	Confirmation
`get_shared_var`	`roomId`, `key`	`{ key, value, found }`
`list_shared_vars`	`roomId`	`{ vars: { … }, count }`

Values are strings. Anything a consumer must reason about structurally (a contract, a spec, a decision) belongs in context, not here.