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/go.opentelemetry.io/otel/propagation/baggage.go

@@ -1,16 +1,5 @@
 // Copyright The OpenTelemetry 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.
+// SPDX-License-Identifier: Apache-2.0
 
 package propagation // import "go.opentelemetry.io/otel/propagation"
 
@@ -31,7 +20,7 @@ type Baggage struct{}
 var _ TextMapPropagator = Baggage{}
 
 // Inject sets baggage key-values from ctx into the carrier.
-func (b Baggage) Inject(ctx context.Context, carrier TextMapCarrier) {
+func (Baggage) Inject(ctx context.Context, carrier TextMapCarrier) {
 	bStr := baggage.FromContext(ctx).String()
 	if bStr != "" {
 		carrier.Set(baggageHeader, bStr)
@@ -39,7 +28,21 @@ func (b Baggage) Inject(ctx context.Context, carrier TextMapCarrier) {
 }
 
 // Extract returns a copy of parent with the baggage from the carrier added.
-func (b Baggage) Extract(parent context.Context, carrier TextMapCarrier) context.Context {
+// If carrier implements [ValuesGetter] (e.g. [HeaderCarrier]), Values is invoked
+// for multiple values extraction. Otherwise, Get is called.
+func (Baggage) Extract(parent context.Context, carrier TextMapCarrier) context.Context {
+	if multiCarrier, ok := carrier.(ValuesGetter); ok {
+		return extractMultiBaggage(parent, multiCarrier)
+	}
+	return extractSingleBaggage(parent, carrier)
+}
+
+// Fields returns the keys who's values are set with Inject.
+func (Baggage) Fields() []string {
+	return []string{baggageHeader}
+}
+
+func extractSingleBaggage(parent context.Context, carrier TextMapCarrier) context.Context {
 	bStr := carrier.Get(baggageHeader)
 	if bStr == "" {
 		return parent
@@ -52,7 +55,23 @@ func (b Baggage) Extract(parent context.Context, carrier TextMapCarrier) context
 	return baggage.ContextWithBaggage(parent, bag)
 }
 
-// Fields returns the keys who's values are set with Inject.
-func (b Baggage) Fields() []string {
-	return []string{baggageHeader}
+func extractMultiBaggage(parent context.Context, carrier ValuesGetter) context.Context {
+	bVals := carrier.Values(baggageHeader)
+	if len(bVals) == 0 {
+		return parent
+	}
+	var members []baggage.Member
+	for _, bStr := range bVals {
+		currBag, err := baggage.Parse(bStr)
+		if err != nil {
+			continue
+		}
+		members = append(members, currBag.Members()...)
+	}
+
+	b, err := baggage.New(members...)
+	if err != nil || b.Len() == 0 {
+		return parent
+	}
+	return baggage.ContextWithBaggage(parent, b)
 }