fix(plugin-system): implement register_driver_type and stop discover() aborting on one bad manifest

- driver/registry.py never actually defined register_driver_type, so
  the plugin-system's 'driver plugin registered successfully' scenario
  was unreachable in production (only simulated via test monkeypatch).
  Added a real implementation wired into the existing driver factory
  registry.
- PluginRegistry.discover() only caught PluginValidationError/
  DuplicatePluginError, so a driver-kind manifest that failed wiring
  (DriverRegistryUnavailableError/PluginTargetResolutionError) aborted
  the whole scan, silently skipping co-located tool/skill manifests.
  Now caught and skipped per manifest instead.

openspec: plugin-system capability, archived change cloud-runtime

cloud/plugins.py

@@ -95,6 +95,19 @@ class PluginTargetResolutionError(RuntimeError):
     """Raised when a plugin target cannot be resolved to a callable."""
 
 
+# Exception types `register()` can raise for a single bad manifest (validation
+# failure, name conflict, or driver-wiring failure). `discover()` catches only
+# these so one bad manifest is skipped/logged, per spec's "malformed manifest
+# file is skipped, not fatal" resilience intent -- anything else propagates as
+# a genuine programmer error.
+_PLUGIN_REGISTRATION_FAILURES = (
+    PluginValidationError,
+    DuplicatePluginError,
+    DriverRegistryUnavailableError,
+    PluginTargetResolutionError,
+)
+
+
 class PluginRegistry:
     """Validates, persists, and wires plugins discovered or submitted directly."""
 
@@ -128,7 +141,7 @@ class PluginRegistry:
             try:
                 self.register(manifest)
                 result.registered.append(manifest)
-            except (PluginValidationError, DuplicatePluginError) as exc:
+            except _PLUGIN_REGISTRATION_FAILURES as exc:
                 result.errors.append(
                     f"entry-point plugin {manifest.name!r}: {exc}"
                 )
@@ -137,7 +150,7 @@ class PluginRegistry:
                 try:
                     self.register(manifest)
                     result.registered.append(manifest)
-                except (PluginValidationError, DuplicatePluginError) as exc:
+                except _PLUGIN_REGISTRATION_FAILURES as exc:
                     result.errors.append(
                         f"manifest-file plugin {manifest.name!r}: {exc}"
                     )