docs: add sota-per-intent principle

[REPPL]

Adds the SOTA per intent principle to the durable record.

Every intent declares the current SOTA — existing alternatives and their rough maturity — then recommends one of three paths, each with its own gate:

1. Build on the SOTA alternative (adopt as dependency) → hard stop: user approval (the standing new-dependency gate, reached through the SOTA declaration).
2. Build a basic native version behind a real seam to adopt SOTA later → proceeds without a stop; the licence to skip approval is keeping adoption open. Default path (the native-floor-plus-opt-in pattern, adr-22/26/25).
3. Bespoke, no viable seam → hard stop: user reviews to set the internal solution's complexity level. A merely deferred seam is still path 2.

Composes with prefer-sota (the adversary→verdict protocol that produces the SOTA read this principle declares). Its next promotion rung is an intent_sota record-lint gate + a ## SOTA intent-template section — noted in the file's Promotion section.

Docs-only; no code change.

Assisted-by: Claude:claude-opus-4-8