EPIC: Full-payload push delivery for Claude Code and Codex agents

Tracks the 2026-09 full-payload inbox delivery refactor across client types. Claude Code is shipped as MSG-413 (shim 2.61.x): full message bodies are delivered through the channel notification, then the hub cursor ACK rides a later poll. Codex uses the separate CLI queue transport tracked by MSG-412. Scope is delivery semantics, deployment/runtime verification, and removal of keystroke-only inbox nudges; it is not a guarantee that a client retained or acted on a handed-off message.