docs: propose governed improvement experiments

[emezac]

Summary

• define the proposed Facility Improvement Lab contract for registry-backed skill and agent-contract experiments
• separate deterministic assignment from receipt-proven exposure
• specify human-gated start, candidate, adoption, and rollback transitions
• define outcome, cost, integrity, and safety guardrails with explicit assessed denominators
• scope the first vertical slice to issue-level assignment and platform evidence, leaving shadow reviewers, guards, model routing, and workflow composition as follow-ups

Why a design PR first

This is a substantial behavior change. The repository contribution rules require its contract to be agreed before implementation. This draft provides a concrete artifact for discussion without adding migrations, routes, runtime behavior, or user-facing claims about an unavailable feature.

The proposal deliberately reuses Registry, receipts, outcomes, gateway attribution, HITL, Watchtower, budgets, and audit. It does not introduce a parallel telemetry system and does not make security/accountability invariants experiment variables.

Closes #188 only when the design is accepted; until then this remains a draft discussion artifact.

Verification

• pnpm --filter @facility/docs build
• node guards/run.mjs
• git diff --check

Open decisions

• whether the first slice should support repository-lane exposure or fail closed to platform-lane runs
• statistical methods and defaults that expose uncertainty honestly
• approval requirements for allocation changes
• whether Registry needs explicit active-version history for rollback proof
• the minimum general issue metadata suitable for cohort eligibility