feat: add data-driven skin/theme engine for CLI customization

Adds a skin system that lets users customize the CLI's visual appearance
through data files (YAML) rather than code changes. Skins define: color
palette, spinner faces/verbs/wings, branding text, and tool output prefix.

New files:
- hermes_cli/skin_engine.py — SkinConfig dataclass, built-in skins
  (default, ares, mono, slate), YAML loader for user skins from
  ~/.hermes/skins/, skin management API
- tests/hermes_cli/test_skin_engine.py — 26 tests covering config,
  built-in skins, user YAML skins, display integration

Modified files:
- agent/display.py — skin-aware spinner wings, faces, verbs, tool prefix
- hermes_cli/banner.py — skin-aware banner colors (title, border, accent,
  dim, text, session) via _skin_color()/_skin_branding() helpers
- cli.py — /skin command handler, skin init from config, skin-aware
  response box label and welcome message
- hermes_cli/config.py — add display.skin default
- hermes_cli/commands.py — add /skin to slash commands

Built-in skins:
- default: classic Hermes gold/kawaii
- ares: crimson/bronze war-god theme (from community PRs #579/#725)
- mono: clean grayscale
- slate: cool blue developer theme

User skins: drop a YAML file in ~/.hermes/skins/ with name, colors,
spinner, branding, and tool_prefix fields. Missing values inherit from
the default skin.

hermes_cli/commands.py

@@ -39,6 +39,8 @@ COMMANDS = {
     "/insights": "Show usage insights and analytics (last 30 days)",
     "/paste": "Check clipboard for an image and attach it",
     "/reload-mcp": "Reload MCP servers from config.yaml",
+    "/rollback": "List or restore filesystem checkpoints (usage: /rollback [number])",
+    "/skin": "Show or change the display skin/theme",
     "/quit": "Exit the CLI (also: /exit, /q)",
 }