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/go.opentelemetry.io/otel/trace/noop/README.md

@@ -0,0 +1,3 @@
+# Trace Noop
+
+[![PkgGoDev](https://pkg.go.dev/badge/go.opentelemetry.io/otel/trace/noop)](https://pkg.go.dev/go.opentelemetry.io/otel/trace/noop)

vendor/go.opentelemetry.io/otel/trace/noop/noop.go

@@ -0,0 +1,112 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// Package noop provides an implementation of the OpenTelemetry trace API that
+// produces no telemetry and minimizes used computation resources.
+//
+// Using this package to implement the OpenTelemetry trace API will effectively
+// disable OpenTelemetry.
+//
+// This implementation can be embedded in other implementations of the
+// OpenTelemetry trace API. Doing so will mean the implementation defaults to
+// no operation for methods it does not implement.
+package noop // import "go.opentelemetry.io/otel/trace/noop"
+
+import (
+	"context"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/codes"
+	"go.opentelemetry.io/otel/trace"
+	"go.opentelemetry.io/otel/trace/embedded"
+)
+
+var (
+	// Compile-time check this implements the OpenTelemetry API.
+
+	_ trace.TracerProvider = TracerProvider{}
+	_ trace.Tracer         = Tracer{}
+	_ trace.Span           = Span{}
+)
+
+// TracerProvider is an OpenTelemetry No-Op TracerProvider.
+type TracerProvider struct{ embedded.TracerProvider }
+
+// NewTracerProvider returns a TracerProvider that does not record any telemetry.
+func NewTracerProvider() TracerProvider {
+	return TracerProvider{}
+}
+
+// Tracer returns an OpenTelemetry Tracer that does not record any telemetry.
+func (TracerProvider) Tracer(string, ...trace.TracerOption) trace.Tracer {
+	return Tracer{}
+}
+
+// Tracer is an OpenTelemetry No-Op Tracer.
+type Tracer struct{ embedded.Tracer }
+
+// Start creates a span. The created span will be set in a child context of ctx
+// and returned with the span.
+//
+// If ctx contains a span context, the returned span will also contain that
+// span context. If the span context in ctx is for a non-recording span, that
+// span instance will be returned directly.
+func (Tracer) Start(ctx context.Context, _ string, _ ...trace.SpanStartOption) (context.Context, trace.Span) {
+	span := trace.SpanFromContext(ctx)
+
+	// If the parent context contains a non-zero span context, that span
+	// context needs to be returned as a non-recording span
+	// (https://github.com/open-telemetry/opentelemetry-specification/blob/3a1dde966a4ce87cce5adf464359fe369741bbea/specification/trace/api.md#behavior-of-the-api-in-the-absence-of-an-installed-sdk).
+	var zeroSC trace.SpanContext
+	if sc := span.SpanContext(); !sc.Equal(zeroSC) {
+		if !span.IsRecording() {
+			// If the span is not recording return it directly.
+			return ctx, span
+		}
+		// Otherwise, return the span context needs in a non-recording span.
+		span = Span{sc: sc}
+	} else {
+		// No parent, return a No-Op span with an empty span context.
+		span = noopSpanInstance
+	}
+	return trace.ContextWithSpan(ctx, span), span
+}
+
+var noopSpanInstance trace.Span = Span{}
+
+// Span is an OpenTelemetry No-Op Span.
+type Span struct {
+	embedded.Span
+
+	sc trace.SpanContext
+}
+
+// SpanContext returns an empty span context.
+func (s Span) SpanContext() trace.SpanContext { return s.sc }
+
+// IsRecording always returns false.
+func (Span) IsRecording() bool { return false }
+
+// SetStatus does nothing.
+func (Span) SetStatus(codes.Code, string) {}
+
+// SetAttributes does nothing.
+func (Span) SetAttributes(...attribute.KeyValue) {}
+
+// End does nothing.
+func (Span) End(...trace.SpanEndOption) {}
+
+// RecordError does nothing.
+func (Span) RecordError(error, ...trace.EventOption) {}
+
+// AddEvent does nothing.
+func (Span) AddEvent(string, ...trace.EventOption) {}
+
+// AddLink does nothing.
+func (Span) AddLink(trace.Link) {}
+
+// SetName does nothing.
+func (Span) SetName(string) {}
+
+// TracerProvider returns a No-Op TracerProvider.
+func (Span) TracerProvider() trace.TracerProvider { return TracerProvider{} }