Fix VS Code MCP server initialization crash and multi-agent architecture

CRITICAL FIX:
- Removed auto-registration during initialization that was causing crashes
- Fixed pattern match error with Inbox.start_link (already_started case)
- Changed architecture to require explicit agent registration with unique IDs

MULTI-AGENT SUPPORT:
- Agents must now register themselves with unique identifiers (e.g., 'Green Platypus')
- register_agent tool now works without requiring prior agent_id
- All other tools require agent_id parameter to identify the calling agent
- Proper error handling for missing agent_id in tool calls

ARCHITECTURE CHANGE:
- One MCP server instance serves multiple agents (as per VS Code design)
- Removed auto-registration of 'GitHub Copilot' agent during initialization
- Each agent must explicitly call register_agent before using other tools

This fixes the VS Code connection error:
MatchError: no match of right hand side value: {:error, {:already_started, #PID<0.215.0>}}

.github/instructions/no-duplicate-files.instructions.md

@@ -8,7 +8,7 @@ applyTo: '**'
 
 **NEVER** create files with adjectives or verbs that duplicate existing functionality:
 - ❌ `enhanced_mcp_server.ex` when `mcp_server.ex` exists
-- ❌ `unified_mcp_server.ex` when `mcp_server.ex` exists  
+- ❌ `unified_mcp_server.ex` when `mcp_server.ex` exists
 - ❌ `mcp_server_manager.ex` when `mcp_server.ex` exists
 - ❌ `new_config.ex` when `config.ex` exists
 - ❌ `improved_task_registry.ex` when `task_registry.ex` exists