Refactor: Change show_completion_notification default to False and remove redundant settings

[szmania]

Summary

This PR refactors the show_completion_notification behavior in Cecli commands:

1. Changes the default value in BaseCommand from True to False
2. Removes redundant show_completion_notification = False declarations from command classes
3. Adds show_completion_notification = True to commands that should show completion notifications
4. Adjusts specific commands based on user feedback to have appropriate notification behavior

Changes Made

• Modified cecli/commands/utils/base_command.py to set show_completion_notification = False as default
• Removed redundant show_completion_notification = False from 34 command classes
• Added show_completion_notification = True to 37 command classes that previously relied on the old default
• Made specific adjustments per user feedback:
  • Set /add, /clear, /diff, /copy, /copy_context, /context-blocks, /context-management, /quit, /spawn-agent, /switch-agent to False
  • Left other informational/action commands as True where appropriate

Motivation

This change simplifies the codebase by removing redundant attribute declarations and makes the default behavior more sensible (most commands don't need to show completion notifications).

Testing

• All modified files have been checked for syntax
• The change follows the existing pattern used in the codebase
• No functional behavior changes for commands that should maintain their notification status

Related Issue

Fixes CLI-44