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/reparse.go

@@ -0,0 +1,131 @@
+//go:build windows
+// +build windows
+
+package winio
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"strings"
+	"unicode/utf16"
+	"unsafe"
+)
+
+const (
+	reparseTagMountPoint = 0xA0000003
+	reparseTagSymlink    = 0xA000000C
+)
+
+type reparseDataBuffer struct {
+	ReparseTag           uint32
+	ReparseDataLength    uint16
+	Reserved             uint16
+	SubstituteNameOffset uint16
+	SubstituteNameLength uint16
+	PrintNameOffset      uint16
+	PrintNameLength      uint16
+}
+
+// ReparsePoint describes a Win32 symlink or mount point.
+type ReparsePoint struct {
+	Target       string
+	IsMountPoint bool
+}
+
+// UnsupportedReparsePointError is returned when trying to decode a non-symlink or
+// mount point reparse point.
+type UnsupportedReparsePointError struct {
+	Tag uint32
+}
+
+func (e *UnsupportedReparsePointError) Error() string {
+	return fmt.Sprintf("unsupported reparse point %x", e.Tag)
+}
+
+// DecodeReparsePoint decodes a Win32 REPARSE_DATA_BUFFER structure containing either a symlink
+// or a mount point.
+func DecodeReparsePoint(b []byte) (*ReparsePoint, error) {
+	tag := binary.LittleEndian.Uint32(b[0:4])
+	return DecodeReparsePointData(tag, b[8:])
+}
+
+func DecodeReparsePointData(tag uint32, b []byte) (*ReparsePoint, error) {
+	isMountPoint := false
+	switch tag {
+	case reparseTagMountPoint:
+		isMountPoint = true
+	case reparseTagSymlink:
+	default:
+		return nil, &UnsupportedReparsePointError{tag}
+	}
+	nameOffset := 8 + binary.LittleEndian.Uint16(b[4:6])
+	if !isMountPoint {
+		nameOffset += 4
+	}
+	nameLength := binary.LittleEndian.Uint16(b[6:8])
+	name := make([]uint16, nameLength/2)
+	err := binary.Read(bytes.NewReader(b[nameOffset:nameOffset+nameLength]), binary.LittleEndian, &name)
+	if err != nil {
+		return nil, err
+	}
+	return &ReparsePoint{string(utf16.Decode(name)), isMountPoint}, nil
+}
+
+func isDriveLetter(c byte) bool {
+	return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')
+}
+
+// EncodeReparsePoint encodes a Win32 REPARSE_DATA_BUFFER structure describing a symlink or
+// mount point.
+func EncodeReparsePoint(rp *ReparsePoint) []byte {
+	// Generate an NT path and determine if this is a relative path.
+	var ntTarget string
+	relative := false
+	if strings.HasPrefix(rp.Target, `\\?\`) {
+		ntTarget = `\??\` + rp.Target[4:]
+	} else if strings.HasPrefix(rp.Target, `\\`) {
+		ntTarget = `\??\UNC\` + rp.Target[2:]
+	} else if len(rp.Target) >= 2 && isDriveLetter(rp.Target[0]) && rp.Target[1] == ':' {
+		ntTarget = `\??\` + rp.Target
+	} else {
+		ntTarget = rp.Target
+		relative = true
+	}
+
+	// The paths must be NUL-terminated even though they are counted strings.
+	target16 := utf16.Encode([]rune(rp.Target + "\x00"))
+	ntTarget16 := utf16.Encode([]rune(ntTarget + "\x00"))
+
+	size := int(unsafe.Sizeof(reparseDataBuffer{})) - 8
+	size += len(ntTarget16)*2 + len(target16)*2
+
+	tag := uint32(reparseTagMountPoint)
+	if !rp.IsMountPoint {
+		tag = reparseTagSymlink
+		size += 4 // Add room for symlink flags
+	}
+
+	data := reparseDataBuffer{
+		ReparseTag:           tag,
+		ReparseDataLength:    uint16(size),
+		SubstituteNameOffset: 0,
+		SubstituteNameLength: uint16((len(ntTarget16) - 1) * 2),
+		PrintNameOffset:      uint16(len(ntTarget16) * 2),
+		PrintNameLength:      uint16((len(target16) - 1) * 2),
+	}
+
+	var b bytes.Buffer
+	_ = binary.Write(&b, binary.LittleEndian, &data)
+	if !rp.IsMountPoint {
+		flags := uint32(0)
+		if relative {
+			flags |= 1
+		}
+		_ = binary.Write(&b, binary.LittleEndian, flags)
+	}
+
+	_ = binary.Write(&b, binary.LittleEndian, ntTarget16)
+	_ = binary.Write(&b, binary.LittleEndian, target16)
+	return b.Bytes()
+}