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.
746d6fd4ccJulius Marminge committed on 9/1/2026, 4:32:53 AMparent8b1f745
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