feat(pluginmgr): add GRPCBroker reverse channel and streaming delivery

[pocky]

Summary

• Completes the bidirectional plugin event system (F092) by activating GRPCBroker to let plugins emit events back to the host at runtime via HostClient, not just as return values from HandleEvent
• Introduces persistent client-side gRPC streaming (StreamEvents RPC) for host-to-plugin event delivery, replacing per-event unary RPCs with a single long-lived connection and automatic fallback to unary when streaming is unavailable
• Promotes hostEventServiceID to an exported constant (HostEventServiceID) shared between host and SDK, eliminating duplicate declarations and the risk of service ID drift
• Extends integration test coverage with 259 lines of broker integration tests (emit/deny/stream/fallback/goroutine-leak/benchmark) to validate the full bidirectional event lifecycle

Changes

Plugin SDK

• pkg/plugin/sdk/host_client.go: Export HostEventServiceID constant (was unexported duplicate); switch to errors.New for static error strings; use exported constant in broker.Dial call
• pkg/plugin/sdk/host_client_test.go: Update assertions to reference exported HostEventServiceID constant

Plugin Manager — Core

• internal/infrastructure/pluginmgr/grpc_event.go: Move domainEventToStreamMessage conversion helper here from stream_manager.go so proto↔domain conversions are co-located in the gRPC adapter file
• internal/infrastructure/pluginmgr/stream_manager.go: Remove domainEventToStreamMessage (relocated to grpc_event.go)
• internal/infrastructure/pluginmgr/rpc_manager.go: Replace local hostEventServiceID constant with sdk.HostEventServiceID to use the single exported source of truth

Plugin Manager — Tests

• internal/infrastructure/pluginmgr/broker_integration_test.go: New file — integration tests covering emit-and-receive, emit denied by manifest, 100-event streaming with sequence numbers, transparent fallback to unary, goroutine leak verification across 50 cycles, F090 backward compatibility, and unary/stream delivery benchmarks
• internal/infrastructure/pluginmgr/grpc_event_test.go: Add unit tests for domainEventToStreamMessage (all fields, zero sequence number)
• internal/infrastructure/pluginmgr/rpc_manager_test.go: Add tests for startBrokerHostService nil-broker/nil-bus no-ops, manifestLookup thread-safety, and stream-manager fallback path; fix wireEventSubscriptions test to use NewStreamManager instead of zero-value struct literal
• internal/infrastructure/pluginmgr/stream_manager_test.go: Replace single lastMessage field with messages slice in mock to support multi-event tests without message clobbering; add dequeue semantics to getLastMessage

Infrastructure — Notify

• internal/infrastructure/notify/webhook_test.go: Replace external httpbin.org URL in concurrent webhook test with local httptest.Server to eliminate network dependency in unit tests

Documentation

• docs/development/plugin-event-architecture.md: Add F092 architecture sections covering GRPCBroker, HostEventService, StreamManager, EventDeliverer seam, fallback handling, and updated ASCII architecture diagram
• docs/user-guide/plugin-events.md: Document HostClient.Emit() as second emission method, add SetHostClient wiring guide, streaming delivery section with automatic fallback description
• README.md: Update Plugin System feature bullet with GRPCBroker reverse channel and persistent streaming delivery descriptions
• CLAUDE.md: Replace two outdated pitfalls/conventions with new rules for gRPC/concurrency coverage requirements and staged-file verification

Test plan

• Run make test-race — verify no data races in stream managers, EventBus delivery goroutines, and broker integration paths
• Run make test-unit — confirm all pluginmgr and sdk unit tests pass including new domainEventToStreamMessage and HostEventServiceID tests
• Run go test ./internal/infrastructure/pluginmgr/ -run TestBrokerIntegration -v — verify emit/deny/stream/fallback/goroutine-leak integration scenarios all pass
• Run go test ./internal/infrastructure/pluginmgr/ -bench BenchmarkEventDelivery -benchmem — confirm stream delivery throughput exceeds unary baseline

Closes #334

---

Generated with awf commit workflow