Add structured denial telemetry

[seanwevans]

Motivation

• Treat denied operations as first-class, structured telemetry so denials can be inspected and segmented rather than inferred from exception strings.
• Record both broker and kernel decisions for denied operations to make policy/bpf interactions observable.
• Surface denial events to monitoring so denial counts and decision-dimensions are available as Prometheus metrics.

Description

• Add a new DenialEvent dataclass and public export via pyisolate.telemetry.DenialEvent. (pyisolate/telemetry.py, exported in pyisolate/__init__.py).
• Attach denial events to PolicyError/PolicyAuthError via a new optional denial_event attribute so callers can inspect the event that caused the raised error. (pyisolate/errors.py).
• Emit structured denial events from sandbox enforcement paths (filesystem, network, subprocess, random, import) and record them on the SandboxThread, plus expose Sandbox.get_denial_events() and SandboxThread.get_denial_events(). (pyisolate/runtime/thread.py, pyisolate/supervisor.py).
• Include denial events in Sandbox.stats and add Prometheus export of denial aggregates and decision-dimensional denial samples in the metrics exporter. (pyisolate/observability/metrics.py).
• Add control-plane denial telemetry when control operations are rejected due to invalid policy tokens. (pyisolate/supervisor.py).
• Document the denial telemetry schema in the protocol docs and add tests validating event payloads and metric export. (docs/protocol.md, tests/test_alerts.py, tests/test_metrics.py).

Testing

• Compiled affected modules with python -m py_compile which succeeded for the modified files.
• Ran focused test groups (tests/test_alerts.py, tests/test_metrics.py, tests/test_capabilities.py, tests/test_policy_enforcement.py) and the denial-telemetry/metering assertions passed.
• Ran additional BPF/ Supervisor unit checks and a broad subset of the suite; the targeted denial telemetry checks pass but the full test suite currently reports 3 failures related to legacy global-state interactions around policy reload monkeypatching and recovery cleanup (test_reload_with_cap, test_refresh_passes_compiled_policy, test_module_import_is_lazy). These failures are orthogonal to the telemetry feature and stem from test environment global-state assumptions.

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Codex Task