CLI-41: Allow agents MCP management + HashPos Base64 + TUI reliability improvements

[szmania]

Summary

This PR enables agents and sub-agents to manage MCP (Model Context Protocol) servers at runtime via new tools and commands (/load-mcp, /remove-mcp, /list-mcp), along with significant infrastructure improvements for event loop safety, HashPos encoding, and TUI reliability.

Key Changes

MCP Server Management (CLI-41)

• New /list-mcp command and ListMcp tool: Lists all loaded and configured MCP servers
• New load_mcp and remove_mcp tools: Allow agents to dynamically load/unload MCP servers at runtime
• Server registration state machine: Per-coder include/exclude sets with hierarchical propagation across agent/sub-agent trees
• Event loop affinity checks: McpServer and HttpBasedMcpServer now track which event loop created their connection and reconnect when the loop changes
• ReloadProgramSignal: New signal type for full program reload after TUI exit

HashPos Encoding Upgrade (Base256 to Base64)

• Migrated from 3-char Base256 to 4-char Base64 content IDs for improved reliability
• Updated all regex patterns, normalization, and resolution logic
• Updated prompt templates with new example IDs

TUI Improvements

• Confirmation queue: Serialized confirmation dialogs prevent overlapping prompts
• Path completion via FileSystemService: Uses trie/trigram indexes for faster filesystem completions
• SwitchCoderSignal handling in worker exception handler
• Version display in TUI banner

Tests

• Comprehensive unit and integration tests for LoadMcpTool and RemoveMcpTool
• Tests for agent and sub-agent MCP management scenarios

[szmania]

@dwash96 this one is ready: #585

A real game changer now folks can put the lists of skills, mcp severs and tools that a agent should use in the agents.md files and the agents will automatically load and unload them