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/proto/encode.go

@@ -5,12 +5,17 @@
 package proto
 
 import (
+	"errors"
+	"fmt"
+
 	"google.golang.org/protobuf/encoding/protowire"
 	"google.golang.org/protobuf/internal/encoding/messageset"
 	"google.golang.org/protobuf/internal/order"
 	"google.golang.org/protobuf/internal/pragma"
 	"google.golang.org/protobuf/reflect/protoreflect"
 	"google.golang.org/protobuf/runtime/protoiface"
+
+	protoerrors "google.golang.org/protobuf/internal/errors"
 )
 
 // MarshalOptions configures the marshaler.
@@ -70,7 +75,32 @@ type MarshalOptions struct {
 	UseCachedSize bool
 }
 
+// flags turns the specified MarshalOptions (user-facing) into
+// protoiface.MarshalInputFlags (used internally by the marshaler).
+//
+// See impl.marshalOptions.Options for the inverse operation.
+func (o MarshalOptions) flags() protoiface.MarshalInputFlags {
+	var flags protoiface.MarshalInputFlags
+
+	// Note: o.AllowPartial is always forced to true by MarshalOptions.marshal,
+	// which is why it is not a part of MarshalInputFlags.
+
+	if o.Deterministic {
+		flags |= protoiface.MarshalDeterministic
+	}
+
+	if o.UseCachedSize {
+		flags |= protoiface.MarshalUseCachedSize
+	}
+
+	return flags
+}
+
 // Marshal returns the wire-format encoding of m.
+//
+// This is the most common entry point for encoding a Protobuf message.
+//
+// See the [MarshalOptions] type if you need more control.
 func Marshal(m Message) ([]byte, error) {
 	// Treat nil message interface as an empty message; nothing to output.
 	if m == nil {
@@ -116,6 +146,9 @@ func emptyBytesForMessage(m Message) []byte {
 
 // MarshalAppend appends the wire-format encoding of m to b,
 // returning the result.
+//
+// This is a less common entry point than [Marshal], which is only needed if you
+// need to supply your own buffers for performance reasons.
 func (o MarshalOptions) MarshalAppend(b []byte, m Message) ([]byte, error) {
 	// Treat nil message interface as an empty message; nothing to append.
 	if m == nil {
@@ -145,12 +178,7 @@ func (o MarshalOptions) marshal(b []byte, m protoreflect.Message) (out protoifac
 		in := protoiface.MarshalInput{
 			Message: m,
 			Buf:     b,
-		}
-		if o.Deterministic {
-			in.Flags |= protoiface.MarshalDeterministic
-		}
-		if o.UseCachedSize {
-			in.Flags |= protoiface.MarshalUseCachedSize
+			Flags:   o.flags(),
 		}
 		if methods.Size != nil {
 			sout := methods.Size(protoiface.SizeInput{
@@ -168,6 +196,10 @@ func (o MarshalOptions) marshal(b []byte, m protoreflect.Message) (out protoifac
 		out.Buf, err = o.marshalMessageSlow(b, m)
 	}
 	if err != nil {
+		var mismatch *protoerrors.SizeMismatchError
+		if errors.As(err, &mismatch) {
+			return out, fmt.Errorf("marshaling %s: %v", string(m.Descriptor().FullName()), err)
+		}
 		return out, err
 	}
 	if allowPartial {