This PR actually fixes two issues:

• the thread title being overridden even after it's been manually edited; as you go and come back from the thread history view, the edited title would get swapped for the auto-summarized one
• the parent thread title sometimes displaying the title of a subagent thread; this would also override the manual edit

• Tests
• Code Reviewed
• Manual QA

Release Notes:

• Agent: Fixed thread titles being overridden even when manually edited.