RFC 007: Durable memory taxonomy & scope

[nkkchem]

Summary

Adds RFC-007: Durable Memory Taxonomy & Scope to contracts/rfcs/, proposing
that the Memory domain grow from its single generic MemoryRecord surface into a
typed taxonomy. Each addition is stated as a non-breaking delta on the current
contract, with an explicit rationale for why the existing surface is insufficient.

Today only episodic memory is meaningfully specified (content: string +
kind: enum). A self-improving, closed-loop scientific platform also needs:

• Semantic — beliefs/facts with confidence, evidence, validity window and
  supersession (belief revision), which an append-only blob cannot express.
• Procedural (highest priority) — replayable recipes; replayProcedure
  materializes a governed RunControl plan (re-passes policy/budget gates, cannot
  escalate privilege). A text blob is not executable.
• Provenance/audit — hash-chained, signed, immutable "why" records; explicitly
  exempt from forget. Current records are mutable/soft-deletable.
• Shared scope — a 3-level memory-scope (agent / campaign / platform)
  replacing the overlapping, unbounded, unenforced sharingMode/sharedWith[].

Working/scratch memory is marked explicitly out of scope (covered by RunControl
checkpoints).

Decision required

§6 (shared/collective scope) needs ratification: adopt the 3-level scope
model, or fall back to agent-only as the conformance floor with
campaign/platform as an optional profile extension.

Notes

• Non-breaking: MemoryRecord and its four ops are unchanged; new surfaces are
  additive. sharingMode/sharedWith[] deprecated (retained for back-compat).
• Origin: ARIAPlatform alignment work.
• This PR adds the RFC document only; schema/OpenAPI changes follow on ratification.