Closes the loop on the blur/close summarize work. Previously the sidebar
always rendered `session.preview` for drafts, so the model call fired by
blur/close updated `session.title` to a value the user never saw, and
then `startExistingSession` summarized again on Start Planning even when
blur had already produced a title for the exact same text.
The sidebar now shows the summarized title once available and falls
back to the inputPayload preview only while the title is still the
placeholder. summarizeDraftTitle persists the plan text it summarized
from via a new `markDraftSummarized` store method, and updateDraft
preserves that marker only while the persisted text still matches it
(text edits invalidate it automatically). startExistingSession reads
the marker and skips its own summarize when the persisted plan equals
what was summarized last, eliminating the redundant model call.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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