Increase the inter-session messaging limit beyond 50

## Problem

The server currently allows only 50 messages between sessions. Complex multi-session workflows can hit this limit quickly, after which coordination messages are not delivered and the user must manually relay them between sessions.

The resulting message is similar to:

> Coordination limitation: session messaging reached its limit, so neither relay was delivered.

## Scenario

1. Query all CI failures from the last week and ask an agent to classify them.
2. Ask the agent to start sessions to fix the top 6-10 issues.
3. Many fixes have overlapping changes, so the sessions need to coordinate frequently.
4. The 50-message limit is reached before the work is complete, blocking further inter-session coordination.

## Request

Increase the number of server-side messages allowed between sessions from 50 to a meaningfully higher value so coordination-heavy workflows can complete without manual message relaying.