feat(plugin): add validators and custom step types (C069)

[pocky]

Summary

• Adds two new plugin capability types: workflow validators (enforce custom validation rules during awf validate) and custom step types (execute new step kinds defined by plugins)
• Introduces ValidatorService and StepTypeService gRPC services with corresponding domain ports (WorkflowValidatorProvider, StepTypeProvider) so third-party plugins can extend the workflow engine without touching core code
• Adds --skip-plugins and --validator-timeout CLI flags to awf run and awf validate, and new --step-types / --validators flags to awf plugin list for capability inspection
• Ships two production-quality example plugins (awf-plugin-database, awf-plugin-security-validator) demonstrating both capabilities end-to-end

Changes

Protocol Buffers

• proto/plugin/v1/plugin.proto: Add ValidatorService and StepTypeService with RegisterStepType, ValidateStepConfig, ValidateWorkflow RPC methods
• proto/plugin/v1/plugin.pb.go: Regenerated protobuf types
• proto/plugin/v1/plugin_grpc.pb.go: Regenerated gRPC stubs
• proto/plugin/v1/plugin_test.go: Tests for new proto messages

Domain

• internal/domain/ports/plugin_validator.go: New WorkflowValidatorProvider and StepTypeProvider port interfaces
• internal/domain/ports/plugin_validator_test.go: Tests for port interfaces and error types
• internal/domain/workflow/step.go: Add Config map[string]any field for plugin step configuration
• internal/domain/workflow/context.go: Extend execution context with plugin step data
• internal/domain/workflow/conversation.go: Minor additions for plugin step support
• internal/domain/workflow/workflow.go: Register StepTypeChecker for plugin-defined step type validation
• internal/domain/pluginmodel/info.go: Add StepTypes []string field to PluginInfo
• internal/domain/pluginmodel/manifest.go: Update CapabilityStepTypes (breaking rename from CapabilityCommands)

Application

• internal/application/execution_service.go: Invoke validator plugins during workflow loading; route plugin step types to their provider at execution time
• internal/application/service.go: Wire ValidatorProvider and StepTypeProvider into the service; expose SetValidatorProvider / SetStepTypeProvider setters

Infrastructure — Plugin Manager

• internal/infrastructure/pluginmgr/grpc_validator.go: gRPC adapter implementing WorkflowValidatorProvider via ValidatorService RPC
• internal/infrastructure/pluginmgr/grpc_step_type.go: gRPC adapter implementing StepTypeProvider via StepTypeService RPC
• internal/infrastructure/pluginmgr/rpc_manager.go: Register and expose validator/step-type adapters per plugin; populate StepTypes on PluginInfo; multi-directory plugin discovery
• internal/infrastructure/pluginmgr/loader.go: Set Type field on loaded plugins

Infrastructure — Repository

• internal/infrastructure/repository/yaml_mapper.go: Map config: block from YAML step to Step.Config
• internal/infrastructure/repository/yaml_types.go: Add Config field to yamlStep

CLI

• internal/interfaces/cli/run.go: Wire --skip-plugins and --validator-timeout flags; register validator/step-type providers into ExecutionService
• internal/interfaces/cli/validate.go: Wire --skip-plugins into validation path
• internal/interfaces/cli/plugin_cmd.go: Add --step-types and --validators flags to awf plugin list; display capability columns
• internal/interfaces/cli/plugins.go: Update plugin list rendering to support new capability columns
• internal/interfaces/cli/config.go: Restrict AWF_PLUGINS_PATH env var to exclusive override

CLI UI

• internal/interfaces/cli/ui/output.go: Add PrintStepTypes and PrintValidators table renderers

Plugin SDK

• pkg/plugin/sdk/validator.go: SDK helpers for implementing ValidatorProvider in plugin binaries
• pkg/plugin/sdk/step_type.go: SDK helpers for implementing StepTypeProvider in plugin binaries
• pkg/plugin/sdk/grpc_plugin.go: Register validator and step-type gRPC servers in Serve()

Interpolation

• pkg/interpolation/resolver.go: Expose step Config map to template resolver

Example Plugins

• examples/plugins/awf-plugin-database/: New plugin implementing SQL query step type with connection pooling
• examples/plugins/awf-plugin-security-validator/: New plugin implementing validator that blocks hardcoded secrets and dangerous commands
• examples/plugins/awf-plugin-echo/main.go, main_test.go, README.md, plugin.yaml: Simplified to use BasePlugin defaults (remove redundant Name/Version overrides)

Tests

• internal/application/service_plugin_step_type_test.go: Tests for step-type routing in ExecutionService
• internal/application/service_plugin_validator_test.go: Tests for validator invocation in ExecutionService
• internal/infrastructure/pluginmgr/grpc_step_type_test.go: Unit tests for gRPC step-type adapter
• internal/infrastructure/pluginmgr/grpc_validator_test.go: Unit tests for gRPC validator adapter
• internal/infrastructure/pluginmgr/rpc_manager_test.go: Updated for multi-directory discovery and new capability queries
• internal/infrastructure/repository/yaml_mapper_config_test.go: Tests for Config field YAML mapping
• internal/infrastructure/repository/yaml_types_config_test.go: Tests for yamlStep.Config parsing
• internal/interfaces/cli/run_plugin_provider_wiring_test.go: Tests verifying validator/step-type providers are wired into the run command
• internal/interfaces/cli/run_skip_plugins_test.go: Tests for --skip-plugins behaviour
• internal/interfaces/cli/validate_skip_plugins_test.go: Tests for --skip-plugins in validate command
• internal/interfaces/cli/arch_lint_config_test.go: Architecture lint rules for new packages
• internal/interfaces/cli/plugin_cmd_test.go, plugins_test.go, plugins_internal_test.go, ui/output_test.go: Updated for new flags and renderers
• internal/domain/workflow/step_config_test.go, step_type_checker_test.go, workflow_plugins_test.go: New domain-level tests
• internal/domain/workflow/context_test.go, conversation_test.go: Extended tests
• pkg/plugin/sdk/validator_test.go, step_type_test.go: SDK unit tests
• examples/plugins/awf-plugin-database/main_test.go, awf-plugin-security-validator/main_test.go: Example plugin unit tests
• pkg/interpolation/resolver_data_test.go: Tests for Config map exposure
• internal/testutil/fixtures/fixtures_test.go: Updated fixture tests
• tests/integration/plugins/plugin_test.go, plugin_validation_manifest_test.go: Updated integration tests

Documentation & Config

• docs/user-guide/plugins.md: Document validator and step-type plugin capabilities with SDK usage examples
• docs/user-guide/workflow-syntax.md: Document config: block on plugin step types
• docs/user-guide/commands.md: Document --skip-plugins, --validator-timeout, --step-types, --validators flags
• CHANGELOG.md: Entry for C069
• README.md, docs/README.md: Update feature list
• tests/fixtures/plugins/valid-full/plugin.yaml: Update fixture for renamed capability_step_types

Test plan

• go test ./internal/... ./pkg/... ./examples/... — all unit tests pass
• Build and install the security-validator example plugin, run awf validate on a workflow containing hardcoded secrets, confirm validation errors are reported
• Define a custom step type in the database plugin, reference it in a workflow YAML with a config: block, run awf run and confirm the plugin step executes
• Run awf plugin list --step-types --validators and confirm capability columns populate for loaded plugins; run with --skip-plugins and confirm plugins are bypassed

Closes #280

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