fix(session): improve session compaction

[nexxeln]

summary

• anchor compaction summaries with a stricter structured prompt and explicit previous-summary updates
• retain split tails when oversized recent turns would otherwise force full summarization
• add regression coverage for repeated compactions, split tails, and compaction pagination boundaries

testing

• bun test test/session/compaction.test.ts test/session/message-v2.test.ts test/session/messages-pagination.test.ts
• bun typecheck

[spaceemotion]

I think this update/change broke compaction for me - the new limit on the compaction text means the AI gets cut off before it can finish properly and is loosing context. 2k chars for a long-running session that has tons of context from before seems way too low (it's usually the file infos that get stripped for me).