Phase 2: Implement Execution Environment Abstraction (v0.3.0)

This commit implements Phase 2 of the CHORUS Task Execution Engine development plan,
providing a comprehensive execution environment abstraction layer with Docker
container sandboxing support.

## New Features

### Core Sandbox Interface
- Comprehensive ExecutionSandbox interface with isolated task execution
- Support for command execution, file I/O, environment management
- Resource usage monitoring and sandbox lifecycle management
- Standardized error handling with SandboxError types and categories

### Docker Container Sandbox Implementation
- Full Docker API integration with secure container creation
- Transparent repository mounting with configurable read/write access
- Advanced security policies with capability dropping and privilege controls
- Comprehensive resource limits (CPU, memory, disk, processes, file handles)
- Support for tmpfs mounts, masked paths, and read-only bind mounts
- Container lifecycle management with proper cleanup and health monitoring

### Security & Resource Management
- Configurable security policies with SELinux, AppArmor, and Seccomp support
- Fine-grained capability management with secure defaults
- Network isolation options with configurable DNS and proxy settings
- Resource monitoring with real-time CPU, memory, and network usage tracking
- Comprehensive ulimits configuration for process and file handle limits

### Repository Integration
- Seamless repository mounting from local paths to container workspaces
- Git configuration support with user credentials and global settings
- File inclusion/exclusion patterns for selective repository access
- Configurable permissions and ownership for mounted repositories

### Testing Infrastructure
- Comprehensive test suite with 60+ test cases covering all functionality
- Docker integration tests with Alpine Linux containers (skipped in short mode)
- Mock sandbox implementation for unit testing without Docker dependencies
- Security policy validation tests with read-only filesystem enforcement
- Resource usage monitoring and cleanup verification tests

## Technical Details

### Dependencies Added
- github.com/docker/docker v28.4.0+incompatible - Docker API client
- github.com/docker/go-connections v0.6.0 - Docker connection utilities
- github.com/docker/go-units v0.5.0 - Docker units and formatting
- Associated Docker API dependencies for complete container management

### Architecture
- Interface-driven design enabling multiple sandbox implementations
- Comprehensive configuration structures for all sandbox aspects
- Resource usage tracking with detailed metrics collection
- Error handling with retryable error classification
- Proper cleanup and resource management throughout sandbox lifecycle

### Compatibility
- Maintains backward compatibility with existing CHORUS architecture
- Designed for future integration with Phase 3 Core Task Execution Engine
- Extensible design supporting additional sandbox implementations (VM, process)

This Phase 2 implementation provides the foundation for secure, isolated task
execution that will be integrated with the AI model providers from Phase 1
in the upcoming Phase 3 development.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

vendor/google.golang.org/protobuf/reflect/protodesc/proto.go

@@ -73,6 +73,16 @@ func ToFileDescriptorProto(file protoreflect.FileDescriptor) *descriptorpb.FileD
 	if syntax := file.Syntax(); syntax != protoreflect.Proto2 && syntax.IsValid() {
 		p.Syntax = proto.String(file.Syntax().String())
 	}
+	if file.Syntax() == protoreflect.Editions {
+		desc := file
+		if fileImportDesc, ok := file.(protoreflect.FileImport); ok {
+			desc = fileImportDesc.FileDescriptor
+		}
+
+		if editionsInterface, ok := desc.(interface{ Edition() int32 }); ok {
+			p.Edition = descriptorpb.Edition(editionsInterface.Edition()).Enum()
+		}
+	}
 	return p
 }
 
@@ -153,6 +163,18 @@ func ToFieldDescriptorProto(field protoreflect.FieldDescriptor) *descriptorpb.Fi
 	if field.Syntax() == protoreflect.Proto3 && field.HasOptionalKeyword() {
 		p.Proto3Optional = proto.Bool(true)
 	}
+	if field.Syntax() == protoreflect.Editions {
+		// Editions have no group keyword, this type is only set so that downstream users continue
+		// treating this as delimited encoding.
+		if p.GetType() == descriptorpb.FieldDescriptorProto_TYPE_GROUP {
+			p.Type = descriptorpb.FieldDescriptorProto_TYPE_MESSAGE.Enum()
+		}
+		// Editions have no required keyword, this label is only set so that downstream users continue
+		// treating it as required.
+		if p.GetLabel() == descriptorpb.FieldDescriptorProto_LABEL_REQUIRED {
+			p.Label = descriptorpb.FieldDescriptorProto_LABEL_OPTIONAL.Enum()
+		}
+	}
 	if field.HasDefault() {
 		def, err := defval.Marshal(field.Default(), field.DefaultEnumValue(), field.Kind(), defval.Descriptor)
 		if err != nil && field.DefaultEnumValue() != nil {