docs(adr): 0003 — one typed daemon contract behind every frontend

[V3RON]

Records ADR 0003. This is the base of a five-PR stack that implements it.

The daemon side is sound — it owns all state and exposes role interfaces. The frontend side is not ready for a fourth client (a programmatic client for agent-device): the socket protocol is the real API but is untyped, HTTP bypasses the protocol and has already diverged on downloads.policy, frontends hold state that belongs to the daemon, and the socket has no roles.

The ADR decides: one contract module declaring every operation once, one dispatcher serving every transport, roles in the daemon, admin authority from a handshake credential, negotiated protocol ranges, one error class with closed codes, and frontends that render the contract and nothing else.

Stack:

1. this PR — the ADR
2. feat/0003-01-contract-module — contract module, range negotiation, daemon validates inputs
3. dispatcher, roles, ownerId, credential handshake, HTTP onto the dispatcher
4. typed client and package exports
5. CLI and MCP onto the client, token operations into the daemon
6. docs, smoke tests, changelog