docs(rfc): explainable PreparedContext receipts

[Tsukikage7]

Which issue or RFC does this PR close?

Closes #1356.

Rationale for this change

POST /v1/context/prepare already selects, cites, and renders bounded context, but the public response is only status, content, and content_bytes. Exact origins stay inside the Runtime and are discarded at the HTTP boundary. Operators, hosts, and evaluation therefore cannot tell what was selected, why items were omitted, or which retrieval path produced the injected bytes.

What changes are included in this PR?

• Add matching English and Chinese RFC documents for PreparedContext Receipts.
• Define an opt-in include_receipt field on the existing prepare operation.
• Define the ephemeral powercontext.prepared-context-receipt.v1 schema, closed omission reasons, privacy bounds, and fail-open assembly.
• Separate PreparedContext Receipts from OpenTelemetry spans and Handoff Receipts.
• Leave OpenAPI, Runtime, CLI, and host plugins unchanged until the RFC is accepted.

Are there any user-facing changes?

No released behavior changes. This PR adds an RFC design only. A later implementation PR would add the optional prepare field and Receipt payload after separate review.

How was this change tested?

• uv run zensical build --clean -s
• uv run prek run --files docs/en/rfcs/1435_explainable_prepared_context_receipts.md docs/zh/rfcs/1435_explainable_prepared_context_receipts.md zensical.toml
• git diff --check
• Cross-checked both locales for matching section structure, policy ID, field names, omission reasons, and bounds.

AI usage statement

Not used.

[CLAassistant]

All committers have signed the CLA.