Migrate to hashbrown::HashMap and add workspace submodules

[WeekendSuperhero]

Summary

Replaces standard library HashMap with hashbrown::HashMap across the codebase for improved performance, and finalizes workspace submodule structure.

Changes

• Dependency: Added hashbrown v0.16 with rayon and serde features to workspace dependencies
• Migration: Updated all std::collections::HashMap usages to hashbrown::HashMap
• Workspace: Configured both agentmail and agentmail-mcp crates to use the shared hashbrown dependency
• Sort order: Cleaned up dependency sort order in Cargo.toml files

Benefits

• Better performance through hashbrown's optimized hashing algorithm
• Parallel processing support via rayon feature for future concurrent operations
• Consistent serialization/deserialization with serde feature

Testing

• Workspace builds successfully with all crates
• No functional changes to existing behavior