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/golang.org/x/sys/cpu/cpu_x86.go

@@ -53,6 +53,9 @@ func initOptions() {
 		{Name: "sse41", Feature: &X86.HasSSE41},
 		{Name: "sse42", Feature: &X86.HasSSE42},
 		{Name: "ssse3", Feature: &X86.HasSSSE3},
+		{Name: "avxifma", Feature: &X86.HasAVXIFMA},
+		{Name: "avxvnni", Feature: &X86.HasAVXVNNI},
+		{Name: "avxvnniint8", Feature: &X86.HasAVXVNNIInt8},
 
 		// These capabilities should always be enabled on amd64:
 		{Name: "sse2", Feature: &X86.HasSSE2, Required: runtime.GOARCH == "amd64"},
@@ -106,7 +109,7 @@ func archInit() {
 		return
 	}
 
-	_, ebx7, ecx7, edx7 := cpuid(7, 0)
+	eax7, ebx7, ecx7, edx7 := cpuid(7, 0)
 	X86.HasBMI1 = isSet(3, ebx7)
 	X86.HasAVX2 = isSet(5, ebx7) && osSupportsAVX
 	X86.HasBMI2 = isSet(8, ebx7)
@@ -134,14 +137,24 @@ func archInit() {
 		X86.HasAVX512VAES = isSet(9, ecx7)
 		X86.HasAVX512VBMI2 = isSet(6, ecx7)
 		X86.HasAVX512BITALG = isSet(12, ecx7)
-
-		eax71, _, _, _ := cpuid(7, 1)
-		X86.HasAVX512BF16 = isSet(5, eax71)
 	}
 
 	X86.HasAMXTile = isSet(24, edx7)
 	X86.HasAMXInt8 = isSet(25, edx7)
 	X86.HasAMXBF16 = isSet(22, edx7)
+
+	// These features depend on the second level of extended features.
+	if eax7 >= 1 {
+		eax71, _, _, edx71 := cpuid(7, 1)
+		if X86.HasAVX512 {
+			X86.HasAVX512BF16 = isSet(5, eax71)
+		}
+		if X86.HasAVX {
+			X86.HasAVXIFMA = isSet(23, eax71)
+			X86.HasAVXVNNI = isSet(4, eax71)
+			X86.HasAVXVNNIInt8 = isSet(4, edx71)
+		}
+	}
 }
 
 func isSet(bitpos uint, value uint32) bool {