refactor: Replace enabled/app_type with is_default for prompts

Major changes:
- Removed 'enabled' and 'app_type' fields from Prompt model
- Added 'is_default' field - only one prompt can be default
- Replaced 'enable/disable' commands with 'set-default/clear-default'
- Updated 'sync' to use comma-separated apps (e.g., -a claude,codex)
- Simplified 'unsync' (removed disable logic)
- Updated 'status' to show default prompt
- Updated all tests for new API

New workflow:
- cam prompt set-default <id>  # Mark a prompt as default
- cam prompt sync              # Sync default to all apps
- cam prompt sync -a claude,codex  # Sync default to specific apps
- cam prompt sync <id> -a gemini   # Sync specific prompt to app

Author: zhujian0805