Fix follow-up conversations for the Droid executor not working

[Asylcreek]

Root Cause

The StandardCodingAgentExecutor trait was updated in commit e442281 to add a reset_to_message_id parameter to the spawn_follow_up method. While the Droid executor had this parameter in its signature (marked as unused), it was still using the old fork_session approach which:

• Created unnecessary copies of session files
• Was incompatible with Droid's actual CLI interface

Implementation Details

The fix simplifies the follow-up flow by:

1. Removing the fork_session() call that created session file copies
2. Directly passing the original session_id to Droid's --session-id flag
3. Letting Droid handle session continuation natively

This aligns with Droid's documented CLI interface where --session-id is used to continue existing sessions.

Droid CLI version is 0.57.9