feat: Add optional name field to file entries and --only filter to get command

[Copilot]

The pepper-sprite.yaml files entries lacked a human-readable identifier, making it impossible to selectively process a single file without editing the config. This adds an optional name field to file entries and a corresponding --only flag on the get command.

Changes

• FileConfig — adds optional name field parsed from YAML; fully backward-compatible (field is omitted if absent)
• get command — adds --only <name> option; when provided, filters the parsed config list to the single matching entry; exits with a usage error if no entry matches
• Tests — covers --only with no matching name and entries lacking a name field

Usage

# pepper-sprite.yaml
files:
  - name: buildings
    fileId: "Z8y1MP7R7z2u31aJq01N"
    output: "assets/images/buildings.png"
  - name: tiles
    fileId: "w7Oy7gJXkBWUJgW7Y1jj"
    output: "assets/images/tiles.png"

# Download and export only the "tiles" entry
pepper_sprite_cli get -k <api-key> --only tiles