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/config.go

@@ -0,0 +1,111 @@
+// Copyright 2016 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 (
+	"time"
+
+	digest "github.com/opencontainers/go-digest"
+)
+
+// ImageConfig defines the execution parameters which should be used as a base when running a container using an image.
+type ImageConfig struct {
+	// User defines the username or UID which the process in the container should run as.
+	User string `json:"User,omitempty"`
+
+	// ExposedPorts a set of ports to expose from a container running this image.
+	ExposedPorts map[string]struct{} `json:"ExposedPorts,omitempty"`
+
+	// Env is a list of environment variables to be used in a container.
+	Env []string `json:"Env,omitempty"`
+
+	// Entrypoint defines a list of arguments to use as the command to execute when the container starts.
+	Entrypoint []string `json:"Entrypoint,omitempty"`
+
+	// Cmd defines the default arguments to the entrypoint of the container.
+	Cmd []string `json:"Cmd,omitempty"`
+
+	// Volumes is a set of directories describing where the process is likely write data specific to a container instance.
+	Volumes map[string]struct{} `json:"Volumes,omitempty"`
+
+	// WorkingDir sets the current working directory of the entrypoint process in the container.
+	WorkingDir string `json:"WorkingDir,omitempty"`
+
+	// Labels contains arbitrary metadata for the container.
+	Labels map[string]string `json:"Labels,omitempty"`
+
+	// StopSignal contains the system call signal that will be sent to the container to exit.
+	StopSignal string `json:"StopSignal,omitempty"`
+
+	// ArgsEscaped
+	//
+	// Deprecated: This field is present only for legacy compatibility with
+	// Docker and should not be used by new image builders.  It is used by Docker
+	// for Windows images to indicate that the `Entrypoint` or `Cmd` or both,
+	// contains only a single element array, that is a pre-escaped, and combined
+	// into a single string `CommandLine`. If `true` the value in `Entrypoint` or
+	// `Cmd` should be used as-is to avoid double escaping.
+	// https://github.com/opencontainers/image-spec/pull/892
+	ArgsEscaped bool `json:"ArgsEscaped,omitempty"`
+}
+
+// RootFS describes a layer content addresses
+type RootFS struct {
+	// Type is the type of the rootfs.
+	Type string `json:"type"`
+
+	// DiffIDs is an array of layer content hashes (DiffIDs), in order from bottom-most to top-most.
+	DiffIDs []digest.Digest `json:"diff_ids"`
+}
+
+// History describes the history of a layer.
+type History struct {
+	// Created is the combined date and time at which the layer was created, formatted as defined by RFC 3339, section 5.6.
+	Created *time.Time `json:"created,omitempty"`
+
+	// CreatedBy is the command which created the layer.
+	CreatedBy string `json:"created_by,omitempty"`
+
+	// Author is the author of the build point.
+	Author string `json:"author,omitempty"`
+
+	// Comment is a custom message set when creating the layer.
+	Comment string `json:"comment,omitempty"`
+
+	// EmptyLayer is used to mark if the history item created a filesystem diff.
+	EmptyLayer bool `json:"empty_layer,omitempty"`
+}
+
+// Image is the JSON structure which describes some basic information about the image.
+// This provides the `application/vnd.oci.image.config.v1+json` mediatype when marshalled to JSON.
+type Image struct {
+	// Created is the combined date and time at which the image was created, formatted as defined by RFC 3339, section 5.6.
+	Created *time.Time `json:"created,omitempty"`
+
+	// Author defines the name and/or email address of the person or entity which created and is responsible for maintaining the image.
+	Author string `json:"author,omitempty"`
+
+	// Platform describes the platform which the image in the manifest runs on.
+	Platform
+
+	// Config defines the execution parameters which should be used as a base when running a container using the image.
+	Config ImageConfig `json:"config,omitempty"`
+
+	// RootFS references the layer content addresses used by the image.
+	RootFS RootFS `json:"rootfs"`
+
+	// History describes the history of each layer.
+	History []History `json:"history,omitempty"`
+}