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/opencontainers/image-spec/specs-go/v1/descriptor.go

@@ -0,0 +1,80 @@
+// Copyright 2016-2022 The Linux Foundation
+//
+// 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 v1
+
+import digest "github.com/opencontainers/go-digest"
+
+// Descriptor describes the disposition of targeted content.
+// This structure provides `application/vnd.oci.descriptor.v1+json` mediatype
+// when marshalled to JSON.
+type Descriptor struct {
+	// MediaType is the media type of the object this schema refers to.
+	MediaType string `json:"mediaType"`
+
+	// Digest is the digest of the targeted content.
+	Digest digest.Digest `json:"digest"`
+
+	// Size specifies the size in bytes of the blob.
+	Size int64 `json:"size"`
+
+	// URLs specifies a list of URLs from which this object MAY be downloaded
+	URLs []string `json:"urls,omitempty"`
+
+	// Annotations contains arbitrary metadata relating to the targeted content.
+	Annotations map[string]string `json:"annotations,omitempty"`
+
+	// Data is an embedding of the targeted content. This is encoded as a base64
+	// string when marshalled to JSON (automatically, by encoding/json). If
+	// present, Data can be used directly to avoid fetching the targeted content.
+	Data []byte `json:"data,omitempty"`
+
+	// Platform describes the platform which the image in the manifest runs on.
+	//
+	// This should only be used when referring to a manifest.
+	Platform *Platform `json:"platform,omitempty"`
+
+	// ArtifactType is the IANA media type of this artifact.
+	ArtifactType string `json:"artifactType,omitempty"`
+}
+
+// Platform describes the platform which the image in the manifest runs on.
+type Platform struct {
+	// Architecture field specifies the CPU architecture, for example
+	// `amd64` or `ppc64le`.
+	Architecture string `json:"architecture"`
+
+	// OS specifies the operating system, for example `linux` or `windows`.
+	OS string `json:"os"`
+
+	// OSVersion is an optional field specifying the operating system
+	// version, for example on Windows `10.0.14393.1066`.
+	OSVersion string `json:"os.version,omitempty"`
+
+	// OSFeatures is an optional field specifying an array of strings,
+	// each listing a required OS feature (for example on Windows `win32k`).
+	OSFeatures []string `json:"os.features,omitempty"`
+
+	// Variant is an optional field specifying a variant of the CPU, for
+	// example `v7` to specify ARMv7 when architecture is `arm`.
+	Variant string `json:"variant,omitempty"`
+}
+
+// DescriptorEmptyJSON is the descriptor of a blob with content of `{}`.
+var DescriptorEmptyJSON = Descriptor{
+	MediaType: MediaTypeEmptyJSON,
+	Digest:    `sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a`,
+	Size:      2,
+	Data:      []byte(`{}`),
+}