Define minimal cell ABI

[seanwevans]

Motivation

• Pin and freeze the public cell surface to a very small, auditable ABI to avoid surface growth and ensure privileged operations go through broker capabilities.
• Make channel events first-class (logs, metrics, broker requests) so the supervisor can treat guest emissions as structured, auditable envelopes.

Description

• Add a versioned minimal ABI in pyisolate/runtime/protocol.py with CellOp, CellABI, MINIMAL_CELL_ABI, and new dataclasses: RecvRequest, PostEvent, LogEvent, MetricEvent, and BrokerRequest, and annotate ExecRequest/CallRequest with op fields.
• Wire guest-facing helpers into the sandbox runtime in pyisolate/runtime/thread.py: log(level, message, **fields), metric(name, value, tags=None), and request(capability, action, payload=None) that emit the new channel event types and enforce capability gating for request(...).
• Small refactors in SandboxThread to centralize emission (_emit) and keep post(...) semantics, and expose log/metric/request in the guest local_vars mapping supplied to executed code.
• Update docs and public API wording to describe the frozen seven-operation surface (exec, call, post, recv, log, metric, request) in docs/ plus API.md and README.md.
• Add and update conformance tests in tests/test_protocol_plane.py to assert the ABI is frozen, validate channel event shapes, and verify capability-gated broker requests.

Testing

• Ran the focused test set: PYTHONPATH=. pytest tests/test_sandbox.py tests/test_protocol_plane.py -q, which passed (20 passed).
• Ran PYTHONPATH=. python -m compileall pyisolate tests/test_protocol_plane.py -q and formatting checks with black --target-version py311 --check on the touched files; these succeeded after applying formatting.
• Note: running the full test suite (PYTHONPATH=. pytest -q) exposed unrelated suite-order/test-double contamination around the test stub for BPFManager.load() (the stub does not accept the mode keyword), which causes many tests to fail; this is orthogonal to the ABI changes and unchanged by this PR.

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Codex Task