Clarify prototype hardening posture

[seanwevans]

Motivation

• The project should not advertise the current release as a hardened runtime when kernel eBPF enforcement and CPython no‑GIL support are still roadmap items.
• Hardened, fail‑closed behavior must be gated by explicit host diagnostics so users do not silently run without the required kernel/tooling.
• Examples and public API docs should avoid implying kernel enforcement for dev/compatibility modes.

Description

• Mark the release as a prototype in README.md, adjust feature wording to label kernel enforcement and no‑GIL support as experimental/roadmap, and update examples to avoid implying enforcement.
• Update API.md and pyproject.toml to reflect prototype status and label the no_gil pytest marker as experimental.
• Add a hardened gate in pyisolate-doctor (pyisolate/doctor.py) that reports missing no‑GIL builds, kernel features, and BPF toolchain availability and returns non‑zero in --mode hardened.
• Report BPF toolchain availability in installation_report() (pyisolate/provenance.py) and wire Supervisor(rollout_mode="hardened") to call assert_hardened_supported() before attempting to load BPF while preserving compatibility with test stubs via a _load_bpf shim in pyisolate/supervisor.py.

Testing

• Compiled affected modules with python -m py_compile pyisolate/doctor.py pyisolate/provenance.py pyisolate/supervisor.py tests/test_provenance.py and the command succeeded.
• Ran targeted test sets pytest -q tests/test_provenance.py tests/test_supervisor.py and pytest -q tests/test_bpf_manager.py, and these targeted suites passed.
• Exercised the doctor CLI with python -m pyisolate.doctor --mode hardened which exited non‑zero on this host as expected because the host does not satisfy hardened requirements.
• The full test-suite (pytest -q) currently exhibits failures unrelated to this documentation/doctor gate change (cross-test BPF manager stubbing and watchdog/thread lifecycle interactions); targeted tests validating this PR pass while the global suite requires addressing test isolation for BPF/watchdog mocks.

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Codex Task