Add volume removal when removing accessories

[cyppe]

Summary

When running kamal accessory remove, directories created via the directories: configuration are deleted from the host, but Docker volumes specified via the volumes: configuration are not removed. This creates an inconsistency in behavior.

Arguably, deleting host-mounted directories is more drastic than deleting Docker-managed volumes, yet only the former happens automatically. This PR adds volume deletion to match the directory deletion behavior, making kamal accessory remove fully clean up all accessory-related storage.

Example Output

Before this change, kamal accessory remove postgres-backup -y would output:

INFO Running docker container stop generate-postgres-backup on 192.168.1.86
INFO Running docker container prune --force --filter label=service=generate-postgres-backup on 192.168.1.86
INFO Running docker image rm --force timescale/timescaledb:latest-pg18 on 192.168.1.86
INFO Running /usr/bin/env rm -rf generate-postgres-backup on 192.168.1.86

After this change, an additional step removes Docker volumes:

INFO Running docker container stop generate-postgres-backup on 192.168.1.86
INFO Running docker container prune --force --filter label=service=generate-postgres-backup on 192.168.1.86
INFO Running docker image rm --force timescale/timescaledb:latest-pg18 on 192.168.1.86
INFO Running /usr/bin/env rm -rf generate-postgres-backup on 192.168.1.86
INFO Running docker volume rm generate-postgres-backups on 192.168.1.86  # <-- NEW

Behavior

Given an accessory configured with:

accessories:
  mysql:
    image: mysql:8.0
    host: 1.1.1.1
    volumes:
      - mysql-data:/var/lib/mysql        # Named volume - will be removed
      - /host/path:/container/path       # Bind mount - ignored (not a Docker volume)
    directories:
      - data:/var/lib/data               # Already removed by existing code

After this change, kamal accessory remove mysql will:

1. Stop the container (existing)
2. Remove the container (existing)
3. Remove the image (existing)
4. Remove the service directory (existing)
5. Remove Docker volumes (docker volume rm mysql-data) - NEW

Only named Docker volumes are removed. Bind mounts (paths starting with /) are correctly excluded since they're not Docker volumes.

Changes

• Add volume_names method to Configuration::Accessory that extracts Docker volume names from the volumes: config (excluding bind mounts which use absolute paths starting with /)
• Add remove_volumes command to Commands::Accessory that runs docker volume rm for the accessory's named volumes
• Add remove_volumes CLI method and call it in remove_accessory
• Update the remove command description and confirmation message to mention volumes
• Add tests for the new functionality