A composer draft is seeded from sticky state, project defaults, prompt links,
the command palette, or the new-thread carry. That seed kept overriding a
started thread's own model and traits, so reopening a thread could silently
switch its model or queue a handoff the user never asked for.

The seed now applies only until the thread starts; afterwards the thread's
selection wins unless the user explicitly picked a model or changed traits.
The same rule gates the provider candidate, effective model and options,
pending handoff, prompt-link context, and new-thread carry. Trait changes now
apply to the model the composer shows, and cleared traits stay cleared rather
than falling back to the thread's options.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>