Pin OpenCode small model in legacy configs

[rasdani]

Summary

• Pin legacy experimental OpenCodeEnv small_model to $OPENAI_MODEL.
• Pin v1 vf.OpenCode small_model to intercepted/model, covering environments/opencode_harbor.

Context

PR #1323 already merged the composable OpenCode harness change. This follow-up covers the remaining affected OpenCode config builders so title/summary sub-agents stay on the intercepted provider instead of falling through to OpenCode's hosted free small model path.

Affected code locations

• verifiers/envs/experimental/opencode_env.py
• verifiers/v1/packages/harnesses/opencode.py

Validation

• uv run ruff check verifiers/envs/experimental/composable/harnesses/opencode.py verifiers/envs/experimental/opencode_env.py verifiers/envs/experimental/opencode_rlm_env.py verifiers/v1/packages/harnesses/opencode.py
• uv run pytest tests/test_opencode_rlm_env.py tests/test_opencode_harbor.py tests/test_v1_harbor_cli.py -q
• push hook: ruff check, ruff format, ty (ci parity)

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Note

Low Risk
Low risk: config-only change that pins OpenCode sub-agent model selection; main behavior change is preventing fallback to OpenCode’s hosted small model, which could affect outputs but not security-critical logic.

Overview
Pins OpenCode’s small_model in legacy config builders so title/summary sub-agents stay on the intercepted provider instead of falling back to OpenCode’s hosted small model.

This sets small_model to $OPENAI_MODEL in OpenCodeEnv.build_opencode_config and to intercepted/model in v1 build_opencode_config (used by the vf.OpenCode harness).

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