Platform v0 alignment RFCs (contracts/rfcs)

[brettin]

Draft RFCs for GMP core-v3 gaps between the specification and ARIAPlatform v0 wire behavior.

Tracked under contracts/rfcs/. Gap IDs reference spec-implementation-gap-register.json.

RFC PRs (merge in this order)

1. RFC 001 — Run invocation I/O and MAG tool_calls (P0)
2. RFC 002 — tool.call events vs LLM tool-use (P0)
3. RFC 005 — Events append/list DTO (P1)
4. RFC 003 — Run status vocabulary (P1)
5. RFC 004 — Budget allocate and enforce (P1)
6. RFC 006 — Capability registry vs spec Capability (P2)

After all RFCs merge, bump external/ARIA in ARIAPlatform and update gap disposition to spec-merged.