v0.6.16

• PFACTORY_EXECUTION_MODEL env override for the contract's execution model (#71 Phase 3). build_execution hardcoded the model per complexity (simple→haiku, standard→sonnet, complex→opus), all of which infer the claude provider — so a signed contract could never tell AIFactory to build on Gemini. Now, when PFACTORY_EXECUTION_MODEL is set (e.g. gemini-2.5-pro), it overrides the per-complexity default; the provider is still inferred from the final model id (gemini-* → antigravity). Unset → unchanged behavior. This lets a PARR run target Gemini end-to-end through the trusted-plan fast path.