Option to regenerate thread titles automatically after each turn

[dshahnaz]

Problem
A thread title is generated early and goes stale as the conversation moves
to other topics. The only way to update it is to regenerate it by hand,
thread by thread.

What I tried
A scheduled task every 5 minutes that calls regenerate_title on open threads.
It works, but it doesn't fit this job:

• Each run creates a new thread and a new scratch folder.
• Each run uses a full agent turn, even if nothing changed.
• It seems to update updatedAt on idle threads, which reorders them and
  makes change detection unreliable.

The t3 CLI has no rename or regenerate command, so a hook or script
can't do this either.

Proposal
A setting to regenerate the title when a turn completes, for example:

• Off (default) / After every turn / Every N turns
• Skip threads the user renamed by hand
• No change to updatedAt or sort order

Alternative: add t3 thread regenerate-title <id> to the CLI, so users can
call it from a Stop hook.