docs(rfc): add Policy subsystem (RFC 0005)

[dvavili]

Summary

Proposes RFC 0005 — Policy Subsystem: promote policy to a first-class gateway subsystem that delegates where policy comes from to a driver, mirroring the subsystem-and-driver model RFC 0001 defines for the gateway (and implements today for compute).

• builtin driver (the default) — today's in-process, store-backed policy path, unchanged.
• Third-party driver — implements a PolicyDriver gRPC contract and runs as an operator-managed process the gateway connects to over a UDS. The gateway does not launch or supervise it.

The change is additive and opt-in per deployment. Enforcement stays the gateway's: projections are verified against a trust store (authentic), admitted all-or-nothing (complete), and gated against mutation (unaltered). A driver's internals — packaging, policy sourcing, remote backends, trust establishment — are out of scope; OpenShell consumes only the projected SandboxPolicy.

Scoping issue: #1713. Related: RFC 0001, RFC 0002, #1703.

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[dvavili]

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