Restructure into workspace crates with full IMAP implementation

[WeekendSuperhero]

Summary

This PR restructures the codebase into a proper Rust workspace with separate crates for the library and MCP server.

Changes

• Workspace structure: Split into agentmail (library) and agentmail-mcp (binary) crates
• IMAP client: Complete implementation with connection pooling (3 sessions/account)
• MCP server: 21 tools and 6 prompts for AI assistant email integration
• Multi-provider support: Gmail, iCloud, Outlook, Yahoo, Fastmail with sensible defaults
• Credential resolution: Password lookup via env vars → config → OS keyring
• Cross-platform: Pure IMAP over TLS, no Mail.app dependency

Technical Details

• Connection pool with semaphore-based concurrency control
• BODY.PEEK throughout (read operations have no side effects)
• HTML to Markdown conversion for LLM-friendly content
• Progress notifications for long-running operations