Resolve named policies before sandbox spawn

[seanwevans]

Motivation

• Ensure public policy strings map to concrete runtime restrictions instead of allowing unconstrained sandboxes when a name/path is provided.
• Provide a single resolver to accept str, dict, compiled policy objects, SandboxPolicy, and Policy inputs for consistent behavior across APIs.

Description

• Add resolve_policy(...) in pyisolate/policy/__init__.py that: resolves file paths and registered public names under policy/, parses compiled or dict policy inputs, converts SandboxPolicy/CompiledPolicy to runtime Policy, and raises PolicyCompilerError for unknown named strings. (pyisolate/policy/__init__.py)
• Wire the resolver into sandbox construction by calling policy = resolve_policy(policy) early in Supervisor.spawn so SandboxThread always receives a runtime Policy object. (pyisolate/supervisor.py)
• Update SDK helpers to resolve policy in the @sandbox decorator and in Pipeline stages before calling spawn. (pyisolate/sdk.py)
• Add repository-level named policy templates under policy/ and document supported public names and resolution semantics in POLICY.md (examples: stdlib.readonly, ml-inference, readonly-fs).
• Export resolve_policy from the package API and include SandboxPolicy/CompiledPolicy types where needed.

Testing

• Ran targeted unit tests: pytest -q tests/test_policy.py::test_resolve_unknown_policy_fails_closed tests/test_policy.py::test_named_policy_applies_runtime_restrictions tests/test_sdk.py and these tests passed.
• Ran a subset: pytest -q tests/test_policy.py tests/test_policy_enforcement.py tests/test_sdk.py and these suites passed.
• Verified bytecode/formatting: python -m py_compile pyisolate/policy/__init__.py pyisolate/supervisor.py pyisolate/sdk.py and ran black on changed files; both succeeded.
• Full test run (pytest -q) was attempted but the run failed due to an unrelated test that injects a stubbed pyisolate.bpf.manager into sys.modules that does not accept the mode= signature used by the supervisor; this is a test-environment interaction and not caused by the resolver changes. All added tests for policy resolution passed.

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Codex Task