Expose no‑GIL readiness axis and add pyisolate doctor CLI

[seanwevans]

Motivation

• Make the runtime no‑GIL readiness a first‑class, visible release axis so deployments can distinguish “parallel cells” (true parallel subinterpreters) from “scheduled compartments” (serialized/scheduled execution) for compatibility and risk decisions.
• Surface the presence and audit status of imported native extensions so CI/fleet tooling can detect when loaded modules block parallel‑cell guarantees.

Description

• Add a new pyisolate/nogil.py module that detects whether CPython was built with --disable-gil, queries the process GIL state when available, enumerates imported native extension modules, accepts PYISOLATE_NOGIL_SAFE_MODULES to mark audited modules as no‑GIL safe, and produces a no_gil_readiness_report() with a parallel_cells vs scheduled_compartments axis.
• Emit a runtime RuntimeWarning on free‑threaded builds when imported native extensions are present but not declared safe, with opt‑out via PYISOLATE_WARN_UNSAFE_NOGIL_EXTENSIONS.
• Extend the doctor tool and provenance output by updating pyisolate/doctor.py, adding a top‑level pyisolate entrypoint in pyisolate/cli.py, and including no_gil in pyisolate/provenance.py so pyisolate-doctor/pyisolate doctor can report gil and extensions (human and --json) information.
• Export no_gil_readiness_report and warn_if_unsafe_native_extensions from the package root and add short documentation to README.md, plus small test coverage additions in tests/test_provenance.py.

Testing

• Ran python -m compileall -q pyisolate successfully.
• Ran focused tests pytest -q tests/test_provenance.py tests/test_sandbox.py::test_spawn_returns_sandbox and observed the targeted tests pass.
• Exercised the new CLI commands with python -m pyisolate.doctor gil and python -m pyisolate.doctor extensions --json and confirmed expected human and JSON output.
• Attempted the full test suite (pytest -q), but the run fails in the current environment with an existing order‑dependent test/stub interaction where _StubBPFManager.load() was mutated earlier in the run and raises TypeError: _StubBPFManager.load() got an unexpected keyword argument 'mode', so full‑suite results are blocked by that unrelated test ordering issue.

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Codex Task