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/Microsoft/go-winio/ea.go

@@ -0,0 +1,137 @@
+package winio
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+)
+
+type fileFullEaInformation struct {
+	NextEntryOffset uint32
+	Flags           uint8
+	NameLength      uint8
+	ValueLength     uint16
+}
+
+var (
+	fileFullEaInformationSize = binary.Size(&fileFullEaInformation{})
+
+	errInvalidEaBuffer = errors.New("invalid extended attribute buffer")
+	errEaNameTooLarge  = errors.New("extended attribute name too large")
+	errEaValueTooLarge = errors.New("extended attribute value too large")
+)
+
+// ExtendedAttribute represents a single Windows EA.
+type ExtendedAttribute struct {
+	Name  string
+	Value []byte
+	Flags uint8
+}
+
+func parseEa(b []byte) (ea ExtendedAttribute, nb []byte, err error) {
+	var info fileFullEaInformation
+	err = binary.Read(bytes.NewReader(b), binary.LittleEndian, &info)
+	if err != nil {
+		err = errInvalidEaBuffer
+		return ea, nb, err
+	}
+
+	nameOffset := fileFullEaInformationSize
+	nameLen := int(info.NameLength)
+	valueOffset := nameOffset + int(info.NameLength) + 1
+	valueLen := int(info.ValueLength)
+	nextOffset := int(info.NextEntryOffset)
+	if valueLen+valueOffset > len(b) || nextOffset < 0 || nextOffset > len(b) {
+		err = errInvalidEaBuffer
+		return ea, nb, err
+	}
+
+	ea.Name = string(b[nameOffset : nameOffset+nameLen])
+	ea.Value = b[valueOffset : valueOffset+valueLen]
+	ea.Flags = info.Flags
+	if info.NextEntryOffset != 0 {
+		nb = b[info.NextEntryOffset:]
+	}
+	return ea, nb, err
+}
+
+// DecodeExtendedAttributes decodes a list of EAs from a FILE_FULL_EA_INFORMATION
+// buffer retrieved from BackupRead, ZwQueryEaFile, etc.
+func DecodeExtendedAttributes(b []byte) (eas []ExtendedAttribute, err error) {
+	for len(b) != 0 {
+		ea, nb, err := parseEa(b)
+		if err != nil {
+			return nil, err
+		}
+
+		eas = append(eas, ea)
+		b = nb
+	}
+	return eas, err
+}
+
+func writeEa(buf *bytes.Buffer, ea *ExtendedAttribute, last bool) error {
+	if int(uint8(len(ea.Name))) != len(ea.Name) {
+		return errEaNameTooLarge
+	}
+	if int(uint16(len(ea.Value))) != len(ea.Value) {
+		return errEaValueTooLarge
+	}
+	entrySize := uint32(fileFullEaInformationSize + len(ea.Name) + 1 + len(ea.Value))
+	withPadding := (entrySize + 3) &^ 3
+	nextOffset := uint32(0)
+	if !last {
+		nextOffset = withPadding
+	}
+	info := fileFullEaInformation{
+		NextEntryOffset: nextOffset,
+		Flags:           ea.Flags,
+		NameLength:      uint8(len(ea.Name)),
+		ValueLength:     uint16(len(ea.Value)),
+	}
+
+	err := binary.Write(buf, binary.LittleEndian, &info)
+	if err != nil {
+		return err
+	}
+
+	_, err = buf.Write([]byte(ea.Name))
+	if err != nil {
+		return err
+	}
+
+	err = buf.WriteByte(0)
+	if err != nil {
+		return err
+	}
+
+	_, err = buf.Write(ea.Value)
+	if err != nil {
+		return err
+	}
+
+	_, err = buf.Write([]byte{0, 0, 0}[0 : withPadding-entrySize])
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// EncodeExtendedAttributes encodes a list of EAs into a FILE_FULL_EA_INFORMATION
+// buffer for use with BackupWrite, ZwSetEaFile, etc.
+func EncodeExtendedAttributes(eas []ExtendedAttribute) ([]byte, error) {
+	var buf bytes.Buffer
+	for i := range eas {
+		last := false
+		if i == len(eas)-1 {
+			last = true
+		}
+
+		err := writeEa(&buf, &eas[i], last)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return buf.Bytes(), nil
+}