[ENHANCEMENT] Restructure package for simplified installation

[burtenshaw]

This PR solves this issue. In short, that the package is not straightforward or conventional to install or import.

DX changes

The PR has a significant amount of new lines, made mostly out of grepping and moving. The fundamental change is this:

User will install like this:

# install everything
pip install openenv
pip install git+https://github.com/meta-pytorch/OpenEnv.git

# install a subset of dependencies
pip install openenv[core]
pip install openenv[cli]

User can use the core library in their envs like so:

from openenv.core.client_types import StepResult
from openenv.core.http_env_client import HTTPEnvClient
from coding_env.models import CodeAction, CodeObservation, CodeState

And install in the env tomls with openenv[core], openenv, or git+https://github.com/meta-pytorch/OpenEnv.git, without a subdirectory reference.

Implementation

The Changes to implement this are:

• Collapsed the repo into a single distributable openenv package: CLI + runtime live under src/openenv/. Following Python conventions and simplifying toml definition.
• openenv_core is now a shim.
• and extras ([core], [cli], [all]) express optional deps in `pyproject.toml.
• Moved sample environments out of src/ into top-level envs/
• updated the CLI template to use openenv.core
• updated docs to import from openenv.core and depend on openenv[core],
• Updated README, guides, scripts, and tests to the new layout