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_darwin.go

@@ -602,6 +602,95 @@ func Connectx(fd int, srcIf uint32, srcAddr, dstAddr Sockaddr, associd SaeAssocI
 	return
 }
 
+const minIovec = 8
+
+func Readv(fd int, iovs [][]byte) (n int, err error) {
+	iovecs := make([]Iovec, 0, minIovec)
+	iovecs = appendBytes(iovecs, iovs)
+	n, err = readv(fd, iovecs)
+	readvRacedetect(iovecs, n, err)
+	return n, err
+}
+
+func Preadv(fd int, iovs [][]byte, offset int64) (n int, err error) {
+	iovecs := make([]Iovec, 0, minIovec)
+	iovecs = appendBytes(iovecs, iovs)
+	n, err = preadv(fd, iovecs, offset)
+	readvRacedetect(iovecs, n, err)
+	return n, err
+}
+
+func Writev(fd int, iovs [][]byte) (n int, err error) {
+	iovecs := make([]Iovec, 0, minIovec)
+	iovecs = appendBytes(iovecs, iovs)
+	if raceenabled {
+		raceReleaseMerge(unsafe.Pointer(&ioSync))
+	}
+	n, err = writev(fd, iovecs)
+	writevRacedetect(iovecs, n)
+	return n, err
+}
+
+func Pwritev(fd int, iovs [][]byte, offset int64) (n int, err error) {
+	iovecs := make([]Iovec, 0, minIovec)
+	iovecs = appendBytes(iovecs, iovs)
+	if raceenabled {
+		raceReleaseMerge(unsafe.Pointer(&ioSync))
+	}
+	n, err = pwritev(fd, iovecs, offset)
+	writevRacedetect(iovecs, n)
+	return n, err
+}
+
+func appendBytes(vecs []Iovec, bs [][]byte) []Iovec {
+	for _, b := range bs {
+		var v Iovec
+		v.SetLen(len(b))
+		if len(b) > 0 {
+			v.Base = &b[0]
+		} else {
+			v.Base = (*byte)(unsafe.Pointer(&_zero))
+		}
+		vecs = append(vecs, v)
+	}
+	return vecs
+}
+
+func writevRacedetect(iovecs []Iovec, n int) {
+	if !raceenabled {
+		return
+	}
+	for i := 0; n > 0 && i < len(iovecs); i++ {
+		m := int(iovecs[i].Len)
+		if m > n {
+			m = n
+		}
+		n -= m
+		if m > 0 {
+			raceReadRange(unsafe.Pointer(iovecs[i].Base), m)
+		}
+	}
+}
+
+func readvRacedetect(iovecs []Iovec, n int, err error) {
+	if !raceenabled {
+		return
+	}
+	for i := 0; n > 0 && i < len(iovecs); i++ {
+		m := int(iovecs[i].Len)
+		if m > n {
+			m = n
+		}
+		n -= m
+		if m > 0 {
+			raceWriteRange(unsafe.Pointer(iovecs[i].Base), m)
+		}
+	}
+	if err == nil {
+		raceAcquire(unsafe.Pointer(&ioSync))
+	}
+}
+
 //sys	connectx(fd int, endpoints *SaEndpoints, associd SaeAssocID, flags uint32, iov []Iovec, n *uintptr, connid *SaeConnID) (err error)
 //sys	sendfile(infd int, outfd int, offset int64, len *int64, hdtr unsafe.Pointer, flags int) (err error)
 
@@ -705,3 +794,7 @@ func Connectx(fd int, srcIf uint32, srcAddr, dstAddr Sockaddr, associd SaeAssocI
 //sys	write(fd int, p []byte) (n int, err error)
 //sys	mmap(addr uintptr, length uintptr, prot int, flag int, fd int, pos int64) (ret uintptr, err error)
 //sys	munmap(addr uintptr, length uintptr) (err error)
+//sys	readv(fd int, iovecs []Iovec) (n int, err error)
+//sys	preadv(fd int, iovecs []Iovec, offset int64) (n int, err error)
+//sys	writev(fd int, iovecs []Iovec) (n int, err error)
+//sys	pwritev(fd int, iovecs []Iovec, offset int64) (n int, err error)