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/containerd/errdefs/pkg/errhttp/http.go

@@ -0,0 +1,96 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+// Package errhttp provides utility functions for translating errors to
+// and from a HTTP context.
+//
+// The functions ToHTTP and ToNative can be used to map server-side and
+// client-side errors to the correct types.
+package errhttp
+
+import (
+	"errors"
+	"net/http"
+
+	"github.com/containerd/errdefs"
+	"github.com/containerd/errdefs/pkg/internal/cause"
+)
+
+// ToHTTP returns the best status code for the given error
+func ToHTTP(err error) int {
+	switch {
+	case errdefs.IsNotFound(err):
+		return http.StatusNotFound
+	case errdefs.IsInvalidArgument(err):
+		return http.StatusBadRequest
+	case errdefs.IsConflict(err):
+		return http.StatusConflict
+	case errdefs.IsNotModified(err):
+		return http.StatusNotModified
+	case errdefs.IsFailedPrecondition(err):
+		return http.StatusPreconditionFailed
+	case errdefs.IsUnauthorized(err):
+		return http.StatusUnauthorized
+	case errdefs.IsPermissionDenied(err):
+		return http.StatusForbidden
+	case errdefs.IsResourceExhausted(err):
+		return http.StatusTooManyRequests
+	case errdefs.IsInternal(err):
+		return http.StatusInternalServerError
+	case errdefs.IsNotImplemented(err):
+		return http.StatusNotImplemented
+	case errdefs.IsUnavailable(err):
+		return http.StatusServiceUnavailable
+	case errdefs.IsUnknown(err):
+		var unexpected cause.ErrUnexpectedStatus
+		if errors.As(err, &unexpected) && unexpected.Status >= 200 && unexpected.Status < 600 {
+			return unexpected.Status
+		}
+		return http.StatusInternalServerError
+	default:
+		return http.StatusInternalServerError
+	}
+}
+
+// ToNative returns the error best matching the HTTP status code
+func ToNative(statusCode int) error {
+	switch statusCode {
+	case http.StatusNotFound:
+		return errdefs.ErrNotFound
+	case http.StatusBadRequest:
+		return errdefs.ErrInvalidArgument
+	case http.StatusConflict:
+		return errdefs.ErrConflict
+	case http.StatusPreconditionFailed:
+		return errdefs.ErrFailedPrecondition
+	case http.StatusUnauthorized:
+		return errdefs.ErrUnauthenticated
+	case http.StatusForbidden:
+		return errdefs.ErrPermissionDenied
+	case http.StatusNotModified:
+		return errdefs.ErrNotModified
+	case http.StatusTooManyRequests:
+		return errdefs.ErrResourceExhausted
+	case http.StatusInternalServerError:
+		return errdefs.ErrInternal
+	case http.StatusNotImplemented:
+		return errdefs.ErrNotImplemented
+	case http.StatusServiceUnavailable:
+		return errdefs.ErrUnavailable
+	default:
+		return cause.ErrUnexpectedStatus{Status: statusCode}
+	}
+}