[Feature] Auto-delete tool progress messages after completion

[phj1081]

Problem

When display.tool_progress is set to new or all, progress messages (e.g. 🔧 terminal, 🔍 search_files) accumulate in the chat on messaging platforms (Discord, Telegram). These are useful while the agent is working, but once the final response is delivered, they become clutter.

Proposed Solution

Add a config option to automatically delete progress messages after the agent completes its response:

display:
  tool_progress: new
  auto_delete_progress: true   # delete progress messages after final response

Behavior

• Progress messages appear as usual during agent execution (single updating message, one line per tool)
• Once the final response is sent, the progress message is deleted
• The final response itself remains
• This keeps the chat clean while still providing real-time feedback during long operations

Current Workaround

Manually patching gateway/run.py to call adapter.delete_message() on the progress message ID after the agent loop completes. This works well but needs to be re-applied on every update.

Additional Context

• The progress message ID is already tracked internally (progress_msg_id / _progress_msg_id_ref)
• delete_message() is already implemented on Discord and Telegram adapters
• This is essentially a few lines of cleanup code, but having it as a built-in config option would be much more maintainable