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/distribution/reference/sort.go

@@ -0,0 +1,75 @@
+/*
+   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 reference
+
+import (
+	"sort"
+)
+
+// Sort sorts string references preferring higher information references.
+//
+// The precedence is as follows:
+//
+//  1. [Named] + [Tagged] + [Digested] (e.g., "docker.io/library/busybox:latest@sha256:<digest>")
+//  2. [Named] + [Tagged]              (e.g., "docker.io/library/busybox:latest")
+//  3. [Named] + [Digested]            (e.g., "docker.io/library/busybo@sha256:<digest>")
+//  4. [Named]                         (e.g., "docker.io/library/busybox")
+//  5. [Digested]                      (e.g., "docker.io@sha256:<digest>")
+//  6. Parse error
+func Sort(references []string) []string {
+	var prefs []Reference
+	var bad []string
+
+	for _, ref := range references {
+		pref, err := ParseAnyReference(ref)
+		if err != nil {
+			bad = append(bad, ref)
+		} else {
+			prefs = append(prefs, pref)
+		}
+	}
+	sort.Slice(prefs, func(a, b int) bool {
+		ar := refRank(prefs[a])
+		br := refRank(prefs[b])
+		if ar == br {
+			return prefs[a].String() < prefs[b].String()
+		}
+		return ar < br
+	})
+	sort.Strings(bad)
+	var refs []string
+	for _, pref := range prefs {
+		refs = append(refs, pref.String())
+	}
+	return append(refs, bad...)
+}
+
+func refRank(ref Reference) uint8 {
+	if _, ok := ref.(Named); ok {
+		if _, ok = ref.(Tagged); ok {
+			if _, ok = ref.(Digested); ok {
+				return 1
+			}
+			return 2
+		}
+		if _, ok = ref.(Digested); ok {
+			return 3
+		}
+		return 4
+	}
+	return 5
+}