docs(plans): preview-model A/B experiment system (opt-in, partner trace export)

[markijbema]

Summary

Adds two planning documents under .plans/ for an upcoming preview/experimental model A/B testing system, in partnership with model providers.

• experimental-models-1.md — Core A/B experiment system: schema, gateway routing, variant picker, admin tRPC + UI, telemetry, API key handling.
• experimental-models-2.md — Partner trace export & replay roadmap (depends on Part 1).

Scope is intentionally narrow:

• This is only for unreleased / preview model checkpoints we are evaluating with provider partners. It is not a general traffic-splitting mechanism for production models.
• It is opt-in only: experimented public model ids are dedicated preview ids (e.g. kilo/preview-experiment-foo) that a user must explicitly select. They are excluded from kilo-auto candidate sets and never silently routed onto a user's behalf. Production model traffic is never bucketed.
• Variant assignment is server-side and blinded from the client; partner exports in Part 2 cover only opt-in preview traffic.

Both plan files now carry a scope banner at the top stating these constraints explicitly.

Verification

N/A — docs only.

Visual Changes

N/A.

Reviewer Notes

• Plans only; no code changes. Looking for feedback on the design before implementation.
• Part 1 is self-contained and the intended first PR. Part 2 is roadmap-level and depends on Part 1's schema.
• Drafted as a PR specifically to make the opt-in / preview-only scope visible early; please flag if the framing needs to be sharper anywhere in the plans.