feat(agent): preserve XHS evidence during compaction

[IRONICBo]

Summary

• Preserve XHS post metadata and bounded question-answer samples in compact tool results.
• Keep oversized XHS payloads valid and artifact-addressable through progressive JSON compaction.
• Bound repeated sawtooth summaries and merge duplicate evidence field by field.
• Preserve opening and closing turn context and document the context lifecycle.

[vercel]

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[IRONICBo]

Closing per maintainer request to keep the active delivery queue focused on four PRs only: audio/video integration, Douyin, TikTok, and the consolidated browser self-healing PR #292. This PR remains available in history and can be restored separately if needed.