Add avocado connect clean command

[nicksinas]

avocado connect clean

New command that reverses what avocado connect init sets up. It removes:

1. connect: section from avocado.yaml (org, project, server_key)
2. avocado-ext-connect-config extension — both the definition in extensions: and
   the - avocado-ext-connect-config entry from the runtime's extensions list
3. overlay/etc/avocado-conn/ directory (contains config.toml with the claim token)

It intentionally preserves avocado-ext-connect and avocado-ext-tunnels — those are
infrastructure extensions present even in projects without connect configuration
(like qemu-quickstart).

CLI flags:

• -r, --runtime <name> — runtime to remove connect-config from (default: dev)
• -C, --config <path> — path to avocado.yaml (default: avocado.yaml)

Files changed:

• src/commands/connect/clean.rs — new ConnectCleanCommand
• src/commands/connect/mod.rs — added pub mod clean
• src/utils/config_edit.rs — added remove_connect_fields(),
  remove_connect_config_extension(), and supporting private functions
  (remove_extension_definition, remove_runtime_extension_entry)
• src/main.rs — added Clean variant to ConnectCommands, import, and dispatch