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/propagation.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"
 
@@ -20,6 +9,7 @@ import (
 )
 
 // TextMapCarrier is the storage medium used by a TextMapPropagator.
+// See ValuesGetter for how a TextMapCarrier can get multiple values for a key.
 type TextMapCarrier interface {
 	// DO NOT CHANGE: any modification will not be backwards compatible and
 	// must never be done outside of a new major release.
@@ -30,7 +20,7 @@ type TextMapCarrier interface {
 	// must never be done outside of a new major release.
 
 	// Set stores the key-value pair.
-	Set(key string, value string)
+	Set(key, value string)
 	// DO NOT CHANGE: any modification will not be backwards compatible and
 	// must never be done outside of a new major release.
 
@@ -40,6 +30,18 @@ type TextMapCarrier interface {
 	// must never be done outside of a new major release.
 }
 
+// ValuesGetter can return multiple values for a single key,
+// with contrast to TextMapCarrier.Get which returns a single value.
+type ValuesGetter interface {
+	// DO NOT CHANGE: any modification will not be backwards compatible and
+	// must never be done outside of a new major release.
+
+	// Values returns all values associated with the passed key.
+	Values(key string) []string
+	// DO NOT CHANGE: any modification will not be backwards compatible and
+	// must never be done outside of a new major release.
+}
+
 // MapCarrier is a TextMapCarrier that uses a map held in memory as a storage
 // medium for propagated key-value pairs.
 type MapCarrier map[string]string
@@ -66,16 +68,27 @@ func (c MapCarrier) Keys() []string {
 	return keys
 }
 
-// HeaderCarrier adapts http.Header to satisfy the TextMapCarrier interface.
+// HeaderCarrier adapts http.Header to satisfy the TextMapCarrier and ValuesGetter interfaces.
 type HeaderCarrier http.Header
 
-// Get returns the value associated with the passed key.
+// Compile time check that HeaderCarrier implements ValuesGetter.
+var _ TextMapCarrier = HeaderCarrier{}
+
+// Compile time check that HeaderCarrier implements TextMapCarrier.
+var _ ValuesGetter = HeaderCarrier{}
+
+// Get returns the first value associated with the passed key.
 func (hc HeaderCarrier) Get(key string) string {
 	return http.Header(hc).Get(key)
 }
 
+// Values returns all values associated with the passed key.
+func (hc HeaderCarrier) Values(key string) []string {
+	return http.Header(hc).Values(key)
+}
+
 // Set stores the key-value pair.
-func (hc HeaderCarrier) Set(key string, value string) {
+func (hc HeaderCarrier) Set(key, value string) {
 	http.Header(hc).Set(key, value)
 }
 
@@ -100,6 +113,8 @@ type TextMapPropagator interface {
 	// must never be done outside of a new major release.
 
 	// Extract reads cross-cutting concerns from the carrier into a Context.
+	// Implementations may check if the carrier implements ValuesGetter,
+	// to support extraction of multiple values per key.
 	Extract(ctx context.Context, carrier TextMapCarrier) context.Context
 	// DO NOT CHANGE: any modification will not be backwards compatible and
 	// must never be done outside of a new major release.