- AGENTS.md: add Skin/Theme System section with architecture, skinnable
elements table, built-in skins list, adding built-in/user skins guide,
YAML example; add skin_engine.py to project structure; mention skin
engine in CLI Architecture section
- CONTRIBUTING.md: add skin_engine.py to project structure; add 'Adding
a Skin/Theme' section with YAML schema, activation instructions
- cli-config.yaml.example: add full skin config documentation with
schema reference, built-in skins list, all color/spinner/branding keys
- docs/skins/example-skin.yaml: complete annotated skin template with
all available fields and inline documentation
- hermes_cli/skin_engine.py: expand module docstring to full schema
reference with all fields documented, usage examples, built-in skins
list
