feat: Add GitHub Copilot SDK backend with MCP and custom tools support

[Copilot]

Description

Implements GitHub Copilot backend integration using github-copilot-sdk with native MCP server support, custom MassGen tools, and stateful session management.

Core Implementation:

• MCP server integration via SDK's native mcpServers configuration
• Custom tool wrapping as MCP server with workspace-aware spec files
• Stateful session management with signature-based cache invalidation
• Authentication flow with retry logic and clear error diagnostics

Key Features:

• Tool routing detection (is_mcp_tool_call, is_custom_tool_call)
• Session signature tracking to recreate sessions on config changes
• Dynamic workspace resolution per agent for custom tool specs
• Permission policy and system message mode configuration
• Tool filtering via allowed_tools/exclude_tools with MCP server integration
• Multimodal tool support (read_media, generate_media)

Architecture:

# Session lifecycle with signature tracking
session_signature = _build_session_signature(
    model=model,
    system_message=system_message,
    workflow_tools=tools,
    mcp_servers=mcp_servers,
    working_directory=working_dir,
    available_tools=available_tools,
)

# Recreate session if config changed
if existing_signature != session_signature:
    await self._destroy_session(agent_id)
    session = await self.client.create_session(session_config)

Tool Integration:

• Workflow tools converted to SDK Tool objects with async handlers
• MCP tools exposed via mcp_servers config with per-server tool filtering
• Custom tools packaged as local MCP server with .copilot/custom_tool_specs.json

Windows Compatibility:

• Fixed read-only file cleanup in _filesystem_manager.py using shutil.rmtree with onerror handler
• Fixed debug logging paths to use tempfile.gettempdir() instead of hardcoded /tmp

Type of change

• New feature (feat:) - Non-breaking change which adds functionality

Checklist

• I have run pre-commit on my changed files and all checks pass
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Pre-commit status

Applied via commit 2a6ce60

How to Test

Prerequisites

# Install Copilot SDK and authenticate
pip install github-copilot-sdk
copilot login  # Follow authentication flow

Test CLI Command

# Basic Copilot backend test
uv run massgen --automation --config massgen/configs/providers/copilot_basic.yaml "What is 2+2?"

# With MCP filesystem tools
uv run massgen --automation --config massgen/configs/providers/copilot_mcp.yaml "List files in current directory"

# With custom tools
uv run massgen --automation --config massgen/configs/providers/copilot_custom_tools.yaml "Search for 'async def' in Python files"

Expected Results

• Copilot backend successfully authenticates via gh CLI
• MCP tools (filesystem, web search) execute correctly
• Custom MassGen tools are accessible via MCP wrapper
• Sessions persist across multiple turns
• Tool calls properly routed (MCP vs workflow vs custom)

Additional context

Dependencies:

• github-copilot-sdk (added to pyproject.toml)

Capabilities:

• Model: gpt-4.1 (default)
• Native MCP support via SDK
• Streaming with reasoning tokens
• Tool execution via SDK handlers
• Session-based state management

Related Files:

• massgen/backend/copilot.py - Main backend implementation
• massgen/backend/capabilities.py - Copilot model registry
• massgen/filesystem_manager/_filesystem_manager.py - Windows cleanup fixes
• massgen/frontend/displays/textual_*.py - Debug logging path fixes

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