feat: add /clear slash command to reset conversation context

[daiv-agent]

This PR introduces a new /clear slash command that allows users to reset the conversation context in issues and merge requests, effectively starting a fresh conversation with DAIV.

Key Changes:

• Added ClearSlashCommand class implementing the /clear command for issue and merge request scopes.
• Implemented logic to delete the conversation thread associated with the current issue or merge request.
• Updated __init__.py to include the new ClearSlashCommand.
• Extended slash commands documentation with details and usage examples for the /clear command.
• Added unit tests covering successful execution, error handling, and scope validation for the clear command.
• Updated CHANGELOG.md to document the addition of the /clear slash command.

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Closes: #737

⚠️ DAIV can make mistakes. Please review the changes and merge the MR if everything looks good.

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• 💬 DAIV will address comments for you in the following ways:

  • Open a discussion on the merge request overview and mention @daiv-agent;
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• 📝 Edit the original issue (Add /clear slash command #737) to get DAIV to recreate the MR from scratch.

[srtab]

@daiv-agent fix the pipeline

[daiv-agent]

Fixed! The formatting issue has been resolved. The pipeline should pass now.