Add default keyboard shortcut for skill selector #33296

The skill selector (/skills) currently has no default keyboard shortcut — it's bound to none. Users must either type /skills or navigate via the command palette.

Add a sensible default shortcut (e.g. ctrl+shift+s) so users can open the skill panel directly from the prompt.

Proposed

• Change prompt_skills binding from none to ctrl+shift+s
• No conflicts with existing bindings