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/github.com/moby/docker-image-spec/specs-go/v1/image.go

@@ -0,0 +1,54 @@
+package v1
+
+import (
+	"time"
+
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+const DockerOCIImageMediaType = "application/vnd.docker.container.image.v1+json"
+
+// DockerOCIImage is a ocispec.Image extended with Docker specific Config.
+type DockerOCIImage struct {
+	ocispec.Image
+
+	// Shadow ocispec.Image.Config
+	Config DockerOCIImageConfig `json:"config,omitempty"`
+}
+
+// DockerOCIImageConfig is a ocispec.ImageConfig extended with Docker specific fields.
+type DockerOCIImageConfig struct {
+	ocispec.ImageConfig
+
+	DockerOCIImageConfigExt
+}
+
+// DockerOCIImageConfigExt contains Docker-specific fields in DockerImageConfig.
+type DockerOCIImageConfigExt struct {
+	Healthcheck *HealthcheckConfig `json:",omitempty"` // Healthcheck describes how to check the container is healthy
+
+	OnBuild []string `json:",omitempty"` // ONBUILD metadata that were defined on the image Dockerfile
+	Shell   []string `json:",omitempty"` // Shell for shell-form of RUN, CMD, ENTRYPOINT
+}
+
+// HealthcheckConfig holds configuration settings for the HEALTHCHECK feature.
+type HealthcheckConfig struct {
+	// Test is the test to perform to check that the container is healthy.
+	// An empty slice means to inherit the default.
+	// The options are:
+	// {} : inherit healthcheck
+	// {"NONE"} : disable healthcheck
+	// {"CMD", args...} : exec arguments directly
+	// {"CMD-SHELL", command} : run command with system's default shell
+	Test []string `json:",omitempty"`
+
+	// Zero means to inherit. Durations are expressed as integer nanoseconds.
+	Interval      time.Duration `json:",omitempty"` // Interval is the time to wait between checks.
+	Timeout       time.Duration `json:",omitempty"` // Timeout is the time to wait before considering the check to have hung.
+	StartPeriod   time.Duration `json:",omitempty"` // The start period for the container to initialize before the retries starts to count down.
+	StartInterval time.Duration `json:",omitempty"` // The interval to attempt healthchecks at during the start period
+
+	// Retries is the number of consecutive failures needed to consider a container as unhealthy.
+	// Zero means inherit.
+	Retries int `json:",omitempty"`
+}