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.

AGENTS.md

@@ -186,6 +186,20 @@ After making ANY code change, you MUST run these commands and fix any issues:
 - **firefly/**: Firefly III API integration
 - Each adapter implements the appropriate port trait
 
+### Commands Module (`banks2ff/src/commands/`)
+- **sync.rs**: Sync command handler
+- **accounts/**: Account management commands
+  - **mod.rs**: Account command dispatch
+  - **link.rs**: Account linking logic and LinkCommands dispatch
+  - **list.rs**: Account listing handler
+  - **status.rs**: Account status handler
+- **transactions/**: Transaction management commands
+  - **mod.rs**: Transaction command dispatch
+  - **list.rs**: Transaction listing handler
+  - **cache.rs**: Cache status handler
+  - **clear.rs**: Cache clearing handler
+- **list.rs**: Source/destination listing handler
+
 ### Client Libraries
 - **gocardless-client/**: Standalone GoCardless API wrapper
 - **firefly-client/**: Standalone Firefly III API wrapper