openenv: default template proj/ path and simplify prompt renderer signatures

[willccbb]

Motivation

• Simplify the default OpenEnv onboarding experience by hiding explicit openenv_project path configuration and assuming the template proj/ layout.
• Make prompt renderer examples minimal and clearer by removing unused **kwargs from example signatures.

Description

• Make OpenEnvEnv.openenv_project optional (openenv_project: str | Path | None = None) and add _resolve_openenv_project to infer proj/ next to the calling environment module with a cwd/proj fallback.
• Update the OpenEnv environment template (verifiers/scripts/init.py) to remove from pathlib import Path, drop **kwargs from the render_prompt example, and stop showing an explicit openenv_project argument.
• Update docs example (verifiers/envs/integrations/README.md) to rely on the default proj/ layout and use the simplified render_prompt(observation) signature.
• Update bundled examples (environments/openenv_echo/*, environments/openenv_textarena/*) to remove unused **kwargs and stop passing openenv_project=Path(__file__).parent / "proj" so they follow the new default behavior.

Testing

• Ran python -m compileall -q verifiers/envs/integrations/openenv_env.py verifiers/scripts/init.py environments/openenv_echo/openenv_echo.py environments/openenv_textarena/openenv_textarena.py which completed successfully.
• Ran ruff check verifiers/envs/integrations/openenv_env.py verifiers/scripts/init.py environments/openenv_echo/openenv_echo.py environments/openenv_textarena/openenv_textarena.py which reported no issues.

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Codex Task

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Note

Medium Risk
Changes how OpenEnvEnv locates the OpenEnv project directory via stack inspection, which could affect runtime path resolution in some call patterns; otherwise this is mostly docs/template/example cleanup.

Overview
OpenEnv environments no longer need to pass an explicit openenv_project path: OpenEnvEnv now accepts openenv_project=None and infers the default proj/ directory next to the calling environment module (falling back to cwd/proj).

OpenEnv onboarding docs and templates were updated to match this default and to simplify prompt_renderer examples by dropping unused **kwargs; bundled example envs (openenv_echo, openenv_textarena) were updated similarly.

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