chore: Move all command handlers to their own files

This makes the code much easier to follow and shortens main.rs from
>1000 lines to around 150.

docs/architecture.md

@@ -10,9 +10,19 @@ Banks2FF implements a **Hexagonal (Ports & Adapters) Architecture** to synchroni
 banks2ff/
 ├── banks2ff/           # Main CLI application
 │   └── src/
+│   ├── commands/   # Command handlers
+│   │   ├── accounts/   # Account management commands
+│   │   │   ├── mod.rs  # Account commands dispatch
+│   │   │   ├── link.rs # Account linking logic
+│   │   │   ├── list.rs # Account listing functionality
+│   │   │   └── status.rs # Account status functionality
+│   │   ├── transactions/ # Transaction management commands
+│   │   ├── list.rs      # Source/destination listing
+│   │   └── sync.rs      # Sync command handler
+│       ├── cli/        # CLI utilities and formatting
 │       ├── core/       # Domain logic and models
 │       ├── adapters/   # External service integrations
-│       └── main.rs     # CLI entry point
+│       └── main.rs     # CLI entry point and command dispatch
 ├── firefly-client/     # Firefly III API client library
 ├── gocardless-client/  # GoCardless API client library
 └── docs/              # Architecture documentation
@@ -49,7 +59,15 @@ banks2ff/
 - `client.rs`: Wrapper for Firefly client for transaction storage
 - Maps domain models to Firefly API format
 
-### 3. API Clients
+### 3. Command Handlers (`banks2ff/src/commands/`)
+
+The CLI commands are organized into focused modules:
+- **sync.rs**: Handles transaction synchronization between sources and destinations
+- **accounts/**: Account management including linking, listing, and status
+- **transactions/**: Transaction inspection, caching, and cache management
+- **list.rs**: Simple listing of available sources and destinations
+
+### 4. API Clients
 
 Both clients are hand-crafted using `reqwest`:
 - Strongly-typed DTOs for compile-time safety