Finalize automatic rewrite behavior per dictation mode

[acgxv]

Context: Automatic rewrite is a configuration toggle, but behavior details are deferred.

Decisions to make:

• Behavior for hold-to-talk vs press-to-toggle dictation.
• When rewrite triggers after completion.
• UI feedback for rewrite in progress and completion.

Reference: docs/spec/ui/app_experience.md

Acceptance:

• Document the final behavior and UI states in the spec.