Allow sandboxes to pass optional entrypoint args and env

[CKwin26]

Summary

• add optional args and env_overrides plumbing to sandbox run_project(...) backends
• forward extra entrypoint arguments through local, Docker, and SSH execution paths
• update the Docker entrypoint wrapper so entrypoint arguments are preserved instead of dropped
• add small regression tests covering local sandbox execution plus Docker/SSH command construction

Why

Some experiment entrypoints need optional runtime parameters or environment overrides to select assets, caches, or execution-time behavior. The sandbox layer previously hard-coded python main.py execution with no clean way to pass extra args or env overrides, which made these cases difficult to support without backend-specific hacks.

This change keeps default behavior unchanged while allowing pipeline callers to pass optional entrypoint arguments and env overrides when needed.

Changes

• extend SandboxProtocol.run_project(...) to accept optional args and env_overrides
• teach ExperimentSandbox to append args to the Python command and merge env overrides into the subprocess environment
• teach DockerSandbox to forward extra args after the entry point and inject -e KEY=value env overrides
• teach SshRemoteSandbox to propagate entry args and env assignments in both bare and Docker-backed execution paths
• make the Docker entrypoint script preserve and forward extra arguments after the entry point
• keep ColabDriveSandbox API-compatible by accepting the new parameters even though the current worker protocol does not yet consume them

Validation

• python -m compileall researchclaw/experiment/sandbox.py researchclaw/experiment/docker_sandbox.py researchclaw/experiment/ssh_sandbox.py researchclaw/experiment/colab_sandbox.py
• bash -n researchclaw/docker/entrypoint.sh
• python -m pytest -q tests/test_entry_point_validation.py -k "passes_args_and_env_overrides"
• python -m pytest -q tests/test_rc_docker_sandbox.py -k "forwards_entry_args_and_env"
• python -m pytest -q tests/test_ssh_and_colab_sandbox.py -k "forwards_args_and_env"

Notes

A few broader Windows-side baseline tests in the repo currently fail for unrelated reasons (for example absolute-path validation expectations and symlink privileges). This PR only relies on the targeted checks above.