docs(rfc): propose unified artifact search

[frf12]

Which issue or RFC does this PR close?

Introduces RFC #1858 for unified Artifact search. Related: #1549 and #1803; database migrations follow #1771. Atomic Memory integration depends on #1809, without blocking other Families.

Rationale for this change

Callers need query-relevant top-N Artifacts rather than unordered entries truncated by a context budget. A common search contract reduces duplicated endpoint and retrieval implementations while preserving Family-specific behavior.

What changes are included in this PR?

This PR contains only the English and Chinese 1858-artifact-search.md RFCs, with no runtime code changes.

• Propose POST /v1/scopes/{scope_id}/artifacts/{family}/search, following the repository RFC template.
• Define Family capability registration, mandatory score normalization, optional retrieval controls, and shared fusion algorithms.
• Keep existing specialized endpoints compatible while requiring integrated Families to call the common internal search method.
• Keep docs(rfc): define artifact search projections and join-free retrieval #1803 as related work and limit the docs(rfc): introduce atomic memory as independent artifacts #1809 dependency to Atomic Memory integration.

Are there any user-facing changes?

Documentation only. This PR proposes a future API and does not change runtime behavior, the OpenAPI contract, or persisted data. Existing specialized endpoints remain available under the proposal.

How was this change tested?

• Read-only bilingual review against the RFC template and agreed scope.
• Checked local Markdown links and matching template headings.
• git diff --cached --check.
• Commit-time repository hooks passed, including ty check.
• Runtime tests and documentation build were not run; this PR changes documentation only.

AI usage statement

Created and reviewed with OpenAI Codex (GPT-6) under maintainer direction.