feat(memory): support configurable summarizer prompt seeds

[cpluss]

Summary

• add configurable summarizer prompt seeding via prompt on both SummarizerConfig and SummarizerOptions
• support three prompt modes: raw system string, prompt/builder seed, or callback factory based on existingSummary
• keep history + summarize/update instructions framework-owned and consistently appended after the seed
• narrow callback context to existingSummary only to avoid accidental history duplication

Tests

• npm test -- tests/dsl/summary.test.ts
• added coverage for string override, callback factory, prompt builder seed, and per-call prompt precedence

[github-actions]

🎉 This PR is included in version 1.7.7 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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