Improve handoff summarizer prompt and previous-state extraction #1034

Follow-up from #1033 and the review thread in Buzz.

#1033 is intentionally scoped to the budget/history bug: the summarizer now receives as much session history as fits in its own model context budget instead of the previous fixed tail/32 KiB cap.

This follow-up tracks the orthogonal summarizer-quality work Dawn/Sami called out:

• revise the handoff summarizer system/user prompt now that it can see much more history
• add/extract a clear # Previous State section so repeated handoffs preserve prior summaries/state intentionally
• add regression coverage around summary structure/state preservation, not just prompt budget construction
• validate with a tiny-context Claude Haiku 4.5 live run and include sample summary output in the PR

Motivation: after #1033, the summarizer has more evidence available, but the existing prompt may still be the bottleneck for producing durable, high-signal handoffs across multiple compactions.