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/google.golang.org/protobuf/internal/errors/errors.go

@@ -17,7 +17,7 @@ var Error = errors.New("protobuf error")
 
 // New formats a string according to the format specifier and arguments and
 // returns an error that has a "proto" prefix.
-func New(f string, x ...interface{}) error {
+func New(f string, x ...any) error {
 	return &prefixError{s: format(f, x...)}
 }
 
@@ -43,7 +43,7 @@ func (e *prefixError) Unwrap() error {
 
 // Wrap returns an error that has a "proto" prefix, the formatted string described
 // by the format specifier and arguments, and a suffix of err. The error wraps err.
-func Wrap(err error, f string, x ...interface{}) error {
+func Wrap(err error, f string, x ...any) error {
 	return &wrapError{
 		s:   format(f, x...),
 		err: err,
@@ -67,7 +67,7 @@ func (e *wrapError) Is(target error) bool {
 	return target == Error
 }
 
-func format(f string, x ...interface{}) string {
+func format(f string, x ...any) string {
 	// avoid "proto: " prefix when chaining
 	for i := 0; i < len(x); i++ {
 		switch e := x[i].(type) {
@@ -87,3 +87,18 @@ func InvalidUTF8(name string) error {
 func RequiredNotSet(name string) error {
 	return New("required field %v not set", name)
 }
+
+type SizeMismatchError struct {
+	Calculated, Measured int
+}
+
+func (e *SizeMismatchError) Error() string {
+	return fmt.Sprintf("size mismatch (see https://github.com/golang/protobuf/issues/1609): calculated=%d, measured=%d", e.Calculated, e.Measured)
+}
+
+func MismatchedSizeCalculation(calculated, measured int) error {
+	return &SizeMismatchError{
+		Calculated: calculated,
+		Measured:   measured,
+	}
+}