[Docs] Update protocol reconnect behavior to match the agent

## Location docs/protocol.md, Connection lifecycle, rule 4 ## Problem The protocol documentation says the current client reconnect default is a fixed 5-second delay. The implemented agent uses exponential backoff capped at 30 seconds with jitter, so the protocol guide is stale. ## Correction Document reconnect after disconnect, re-registration, exponential backoff, a 30-second cap, and jitter. Do not promise an exact delay because jitter makes it variable. ## Acceptance criteria - The protocol no longer describes reconnect as a fixed 5-second delay. - The documented behavior matches the agent implementation. - Re-registration after reconnect remains documented. - Documentation-only change; no agent code changes.