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/internal/impl/message.go

@@ -35,7 +35,7 @@ type MessageInfo struct {
 	Exporter exporter
 
 	// OneofWrappers is list of pointers to oneof wrapper struct types.
-	OneofWrappers []interface{}
+	OneofWrappers []any
 
 	initMu   sync.Mutex // protects all unexported fields
 	initDone uint32
@@ -47,7 +47,7 @@ type MessageInfo struct {
 // exporter is a function that returns a reference to the ith field of v,
 // where v is a pointer to a struct. It returns nil if it does not support
 // exporting the requested field (e.g., already exported).
-type exporter func(v interface{}, i int) interface{}
+type exporter func(v any, i int) any
 
 // getMessageInfo returns the MessageInfo for any message type that
 // is generated by our implementation of protoc-gen-go (for v2 and on).
@@ -201,7 +201,7 @@ fieldLoop:
 	}
 	for _, fn := range methods {
 		for _, v := range fn.Func.Call([]reflect.Value{reflect.Zero(fn.Type.In(0))}) {
-			if vs, ok := v.Interface().([]interface{}); ok {
+			if vs, ok := v.Interface().([]any); ok {
 				oneofWrappers = vs
 			}
 		}
@@ -256,7 +256,7 @@ func (mi *MessageInfo) Message(i int) protoreflect.MessageType {
 
 type mapEntryType struct {
 	desc    protoreflect.MessageDescriptor
-	valType interface{} // zero value of enum or message type
+	valType any // zero value of enum or message type
 }
 
 func (mt mapEntryType) New() protoreflect.Message {