docs(runtime): outline receipt export boundary

[cyq1017]

Problem

• Proposal: read-only conformance receipt export for runtime thread/turn replay #2556 proposes a local read-only receipt export for completed runtime turns, but the contract needs a small docs-first boundary before code lands.
• The current runtime API docs list threads, turns, approvals, and events, but do not describe what a future receipt may claim or must avoid claiming.

Change

• Add docs/RECEIPTS.md with non-goals, candidate local surfaces, current durable data sources, a draft JSON shape, and deterministic builder rules.
• Link the protocol note from docs/RUNTIME_API.md and mark the receipt endpoint as proposed only.

Verification

• git diff --cached --check
• diff secret scan; only the existing safety phrase "secret leakage" appeared as a false-positive match
• Manual cross-check against ThreadRecord, TurnRecord, TurnItemRecord, and RuntimeEventRecord

Refs #2556

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