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/golang.org/x/sys/unix/syscall_solaris.go

@@ -1102,3 +1102,90 @@ func (s *Strioctl) SetInt(i int) {
 func IoctlSetStrioctlRetInt(fd int, req int, s *Strioctl) (int, error) {
 	return ioctlPtrRet(fd, req, unsafe.Pointer(s))
 }
+
+// Ucred Helpers
+// See ucred(3c) and getpeerucred(3c)
+
+//sys	getpeerucred(fd uintptr, ucred *uintptr) (err error)
+//sys	ucredFree(ucred uintptr) = ucred_free
+//sys	ucredGet(pid int) (ucred uintptr, err error) = ucred_get
+//sys	ucredGeteuid(ucred uintptr) (uid int) = ucred_geteuid
+//sys	ucredGetegid(ucred uintptr) (gid int) = ucred_getegid
+//sys	ucredGetruid(ucred uintptr) (uid int) = ucred_getruid
+//sys	ucredGetrgid(ucred uintptr) (gid int) = ucred_getrgid
+//sys	ucredGetsuid(ucred uintptr) (uid int) = ucred_getsuid
+//sys	ucredGetsgid(ucred uintptr) (gid int) = ucred_getsgid
+//sys	ucredGetpid(ucred uintptr) (pid int) = ucred_getpid
+
+// Ucred is an opaque struct that holds user credentials.
+type Ucred struct {
+	ucred uintptr
+}
+
+// We need to ensure that ucredFree is called on the underlying ucred
+// when the Ucred is garbage collected.
+func ucredFinalizer(u *Ucred) {
+	ucredFree(u.ucred)
+}
+
+func GetPeerUcred(fd uintptr) (*Ucred, error) {
+	var ucred uintptr
+	err := getpeerucred(fd, &ucred)
+	if err != nil {
+		return nil, err
+	}
+	result := &Ucred{
+		ucred: ucred,
+	}
+	// set the finalizer on the result so that the ucred will be freed
+	runtime.SetFinalizer(result, ucredFinalizer)
+	return result, nil
+}
+
+func UcredGet(pid int) (*Ucred, error) {
+	ucred, err := ucredGet(pid)
+	if err != nil {
+		return nil, err
+	}
+	result := &Ucred{
+		ucred: ucred,
+	}
+	// set the finalizer on the result so that the ucred will be freed
+	runtime.SetFinalizer(result, ucredFinalizer)
+	return result, nil
+}
+
+func (u *Ucred) Geteuid() int {
+	defer runtime.KeepAlive(u)
+	return ucredGeteuid(u.ucred)
+}
+
+func (u *Ucred) Getruid() int {
+	defer runtime.KeepAlive(u)
+	return ucredGetruid(u.ucred)
+}
+
+func (u *Ucred) Getsuid() int {
+	defer runtime.KeepAlive(u)
+	return ucredGetsuid(u.ucred)
+}
+
+func (u *Ucred) Getegid() int {
+	defer runtime.KeepAlive(u)
+	return ucredGetegid(u.ucred)
+}
+
+func (u *Ucred) Getrgid() int {
+	defer runtime.KeepAlive(u)
+	return ucredGetrgid(u.ucred)
+}
+
+func (u *Ucred) Getsgid() int {
+	defer runtime.KeepAlive(u)
+	return ucredGetsgid(u.ucred)
+}
+
+func (u *Ucred) Getpid() int {
+	defer runtime.KeepAlive(u)
+	return ucredGetpid(u.ucred)
+}