P2: docs/future/context-receipt-impact-trace: decide keep-or-remove for the completed implementation-plan doc

[macanderson]

Deferred from cleanup PR #47 as a judgment call.

docs/future/context-receipt-impact-trace/context-receipt-impact-trace-plan.md is a finished agentic implementation plan, not a maintained design doc:

• It uses checkbox task tracking (- [ ]) and embeds an internal directive ("REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development").
• It names a stack ("Vinext/Vite, Cloudflare Workers") that doesn't match the actual Next.js/Fumadocs site — i.e. a stale planning artifact.
• The feature it plans already shipped: site/src/app/_components/ContextReceiptDemo.tsx + site/tests/context-receipt.test.mjs.

Its sibling context-receipt-impact-trace-design.md is referenced (from docs/sketches/host-trace.md) and should stay.

Ask: keep the plan for provenance, or remove it? If the docs/future/ area is meant to hold living forward-looking design (not completed checklists), this one has outlived its purpose.

https://claude.ai/code/session_01Dntx9eZ8zQRLb1MuNabrCp

[macanderson]

Re-verified 2026-07-26 against origin/main @ 1ee9b1c.

All four claims in the body verify. The implementation-plan doc is still present, the feature it plans is genuinely shipped (a site component, with tests), and the sibling design doc is genuinely referenced from docs/sketches/host-trace.md — so the two docs are not interchangeable and cannot be removed together.

Recommended scope: keep, but resolve inline — this is a two-minute decision blocking nothing, and it costs more to carry than to make. Suggested resolution: remove the completed implementation-plan doc, keep the design doc that is still referenced.