docs: add opt-in native-state contract profile

[ianalitis]

Summary

Refs #1.

Adds a small, opt-in contract-only profile for hosts that already own task state, verification receipts, budgets, and approvals.

• Bind the contract to existing host tasks and receipts instead of creating .gloop/ files, copied state, or another runner/resume mechanism.
• Require a finite budget, one scoped improvement per iteration, verified goal-before-refinement, and preserved failure, no-gain, operator, and approval stops.
• Clarify that this profile requires explicit nonempty safety boundaries, while leaving the base schema and existing contracts unchanged.
• Add a short reference in the generic adapter. Other harness bundles, installers, and runtime code are untouched.

Verification

• git diff-tree --check HEAD^ HEAD
• python3 -m json.tool core/contract.schema.json
• Confirmed exactly two changed documents and a byte-identical schema against b151e2a98caf018a003784deb47f454251bb01df.
• Reviewed the incomplete-goal, failed-gate, exhausted-budget, no-action, and approval-required cases for consistency.

These are documentation/static checks, not runtime enforcement or demonstrated host conformance. No installation, package build, or model benchmark was performed. The profile is selected in request/adapter prose, not through a new schema field or loop-mode value.