feat(commands): Complete quick peek providers dynamically

[mhiro2]

Summary

• Replace hardcoded provider list with dynamic completion using registered providers
• Add list_providers() API to expose currently registered provider names
• Update tests to verify dynamic provider completion behavior

Changes

• 643411a : feat(commands): complete quick peek providers dynamically
  • Add list_providers() function to return sorted provider names
  • Update PeekstackQuickPeek command completion to use dynamic provider list
• 71b0d7a : test(commands): verify dynamic quick peek completion
  • Test that completion reflects enabled/disabled providers based on config
  • Test that custom providers registered via register_provider() appear in completion