docs: state portable handoff limits

Explain the eligible timeline items, whitespace-normalized 240-character prefixes, omitted tail risk, and practical preparation for provider or fork handoffs. Distinguish this rule from the legacy import's 32,000-character transcript suffix.

Audit: D04

Written by GPT-5.6 Sol with Codex.
01fb9b2f8bJulius Marminge committed on 9/5/2026, 4:54:28 AMparentb991834
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