[Feature]: Cross-session fair queueing plus per-channel in-flight caps make multi-tenant deployment a first-class concern

### Problem Current state task_runtime defaults: [task_runtime] max_concurrency = 4 max_pending_per_session = 64 The full design surfaces across the [0.1.0-alpha.1] CHANGELOG entries: - "Tool concurrency: same-turn safe tool_calls dispatched concurrently via asyncio.gather" - "Per-channel-adapter in-flight reply cap (_ChannelInFlightSet) so a single channel cannot exhaust the global concurrency budget" ...