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/impl/encode.go

@@ -49,8 +49,11 @@ func (mi *MessageInfo) sizePointer(p pointer, opts marshalOptions) (size int) {
 		return 0
 	}
 	if opts.UseCachedSize() && mi.sizecacheOffset.IsValid() {
-		if size := atomic.LoadInt32(p.Apply(mi.sizecacheOffset).Int32()); size >= 0 {
-			return int(size)
+		// The size cache contains the size + 1, to allow the
+		// zero value to be invalid, while also allowing for a
+		// 0 size to be cached.
+		if size := atomic.LoadInt32(p.Apply(mi.sizecacheOffset).Int32()); size > 0 {
+			return int(size - 1)
 		}
 	}
 	return mi.sizePointerSlow(p, opts)
@@ -60,7 +63,7 @@ func (mi *MessageInfo) sizePointerSlow(p pointer, opts marshalOptions) (size int
 	if flags.ProtoLegacy && mi.isMessageSet {
 		size = sizeMessageSet(mi, p, opts)
 		if mi.sizecacheOffset.IsValid() {
-			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), int32(size))
+			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), int32(size+1))
 		}
 		return size
 	}
@@ -84,13 +87,16 @@ func (mi *MessageInfo) sizePointerSlow(p pointer, opts marshalOptions) (size int
 		}
 	}
 	if mi.sizecacheOffset.IsValid() {
-		if size > math.MaxInt32 {
+		if size > (math.MaxInt32 - 1) {
 			// The size is too large for the int32 sizecache field.
 			// We will need to recompute the size when encoding;
 			// unfortunately expensive, but better than invalid output.
-			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), -1)
+			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), 0)
 		} else {
-			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), int32(size))
+			// The size cache contains the size + 1, to allow the
+			// zero value to be invalid, while also allowing for a
+			// 0 size to be cached.
+			atomic.StoreInt32(p.Apply(mi.sizecacheOffset).Int32(), int32(size+1))
 		}
 	}
 	return size
@@ -149,6 +155,14 @@ func (mi *MessageInfo) marshalAppendPointer(b []byte, p pointer, opts marshalOpt
 	return b, nil
 }
 
+// fullyLazyExtensions returns true if we should attempt to keep extensions lazy over size and marshal.
+func fullyLazyExtensions(opts marshalOptions) bool {
+	// When deterministic marshaling is requested, force an unmarshal for lazy
+	// extensions to produce a deterministic result, instead of passing through
+	// bytes lazily that may or may not match what Go Protobuf would produce.
+	return opts.flags&piface.MarshalDeterministic == 0
+}
+
 func (mi *MessageInfo) sizeExtensions(ext *map[int32]ExtensionField, opts marshalOptions) (n int) {
 	if ext == nil {
 		return 0
@@ -158,6 +172,14 @@ func (mi *MessageInfo) sizeExtensions(ext *map[int32]ExtensionField, opts marsha
 		if xi.funcs.size == nil {
 			continue
 		}
+		if fullyLazyExtensions(opts) {
+			// Don't expand the extension, instead use the buffer to calculate size
+			if lb := x.lazyBuffer(); lb != nil {
+				// We got hold of the buffer, so it's still lazy.
+				n += len(lb)
+				continue
+			}
+		}
 		n += xi.funcs.size(x.Value(), xi.tagsize, opts)
 	}
 	return n
@@ -176,6 +198,13 @@ func (mi *MessageInfo) appendExtensions(b []byte, ext *map[int32]ExtensionField,
 		var err error
 		for _, x := range *ext {
 			xi := getExtensionFieldInfo(x.Type())
+			if fullyLazyExtensions(opts) {
+				// Don't expand the extension if it's still in wire format, instead use the buffer content.
+				if lb := x.lazyBuffer(); lb != nil {
+					b = append(b, lb...)
+					continue
+				}
+			}
 			b, err = xi.funcs.marshal(b, x.Value(), xi.wiretag, opts)
 		}
 		return b, err
@@ -191,6 +220,13 @@ func (mi *MessageInfo) appendExtensions(b []byte, ext *map[int32]ExtensionField,
 		for _, k := range keys {
 			x := (*ext)[int32(k)]
 			xi := getExtensionFieldInfo(x.Type())
+			if fullyLazyExtensions(opts) {
+				// Don't expand the extension if it's still in wire format, instead use the buffer content.
+				if lb := x.lazyBuffer(); lb != nil {
+					b = append(b, lb...)
+					continue
+				}
+			}
 			b, err = xi.funcs.marshal(b, x.Value(), xi.wiretag, opts)
 			if err != nil {
 				return b, err