sepiaAgent sessions, agnostically

One canonical IR, store drivers, and an ACP client for every coding agent.

Canonical session IR

Every agent's session store maps onto one intermediate representation — one model for titles, history, tool calls, and restores.

Per-agent adapter binaries speak ndjson JSON-RPC over stdio. Add an agent without touching sepia's core.

sepia-session owns the mechanism — attach, prompt, cancel, permissions, restore/rewind, liveness probing. Orchestration policy lives in peregrin.