BackendRegistry / OPAPolicyBackend / CedarPolicyBackend are never consulted by govern() - registering a backend is a silent no-op

**What's misleading**

`agentmesh/governance/backend.py` documents `BackendRegistry` as the way to plug in external policy engines (OPA, Cedar, custom):

```python
# Register a backend
BackendRegistry.register(my_opa_evaluator)

# Retrieve by name
backend = BackendRegistry.get("opa")
decision = backend.evaluate("export", {"agent": {"role": "analyst"}})
```

and `OPAPolicyBackend`/`CedarPolicyBackend` both exist and correctly implement the `ExternalPolicyBackend` protocol. Reasonable to assume that once registered, `govern()`'s own policy evaluation would use a registered backend - the module's own docstring says backends are "used interchangeably by the governance layer."

**What actually happens**

`govern()`'s enforcement path (`govern.py`'s `GovernedCallable.__call__` → `PolicyEngine.evaluate` in `policy_evaluator.py`... actually via `self._engine.evaluate(self._config.agent_id, context)`) never references `agentmesh.governance.backend` or `BackendRegistry` at all - confirmed with `grep -rl BackendRegistry` across the whole `governance/` package: only `backend.py`, `opa.py`, `cedar.py`, and `__init__.py` (the re-export) reference it. Registering a backend with `BackendRegistry.register(...)` has no effect on what `govern()` does; its `Policy`/`PolicyRule` YAML/JSON engine is the only thing that runs, always.

**Impact**

Anyone reaching for OPA/Cedar because the built-in YAML condition DSL can't express what they need (see also: it's regex-based and doesn't support field-vs-field comparisons - happy to file that separately if useful) will reasonably try `BackendRegistry.register()` first, based on the docstring, and get silent no-op behavior - no error, the governed call just keeps using the YAML engine's `default_action`.

**Suggested fix / ask**

Either:
- Have `govern()`/`PolicyEngine` actually consult `BackendRegistry` when a policy rule specifies an external backend (e.g. a `backend: opa` field on `PolicyRule`), or
- Update `backend.py`'s docstring and the OPA/Cedar module docs to make clear this is a standalone, manually-wired API - not something `govern()` picks up automatically.

Happy to share the minimal repro (a two-file example showing `BackendRegistry.register()` having zero effect on a `govern()`-wrapped call) if useful.

Start with govern.py's GovernedCallable.__call__ and policy_evaluator.py's PolicyEngine.evaluate, then compare that path with BackendRegistry in backend.py and the integrations in opa.py and cedar.py. Determine whether the intended behavior is explicit backend selection or documentation-only standalone use; done means registered backends no longer silently mislead users and the relevant behavior or documentation is covered by tests.

Written by the indexing model from the issue text.