docs(architecture): audit Model access slice under #3522

[bytelazy]

Summary

Audits the Model access subsystem slice under #3522 — the two documents listed there were read against current main and either corrected or confirmed accurate.

• docs/architecture/openai-responses-incremental-transport.md — verified accurate, no change. Every cited symbol (ModelAdapter, endLane, previous_response_id, prompt_cache_key, the sdk.response.messages contrast) still exists with the semantics the document describes.
• docs/model-metadata-firstscreen-optimization.md — the optimization it proposes has been implemented. The renderer startup path no longer statically imports model-metadata; OnboardingHero no longer imports RECOMMENDED_PROVIDER_TYPES at runtime; PROVIDER_REGISTRY is now reached only from lazy Settings paths — matching the document's desired outcome. The document was being read as an open proposal when its acceptance criteria are met, so it is moved to docs/archive/ with an Archived: banner so it is read as the historical design record.

The move is a git rename (history preserved); the only content change is the two-line banner. docs/archive/** is out of scope for #3522's audit, but moving a now-implemented proposal into the archive is the audit conclusion for this document, not an edit to an existing archive entry.

Refs #3522

Verification

• Confirmed no script, test, or other document references model-metadata-firstscreen-optimization by path, so the move breaks no links (checked scripts/, .github/, all *.md/*.ts/*.mjs).
• Ran node --test scripts/source-legal-inventory.test.mjs (the test that reads docs/): 5/5 pass.
• Confirmed the cited symbols in the accurate document still exist via grep against packages/runtime/src.
• Git recognizes the operation as a rename (R), so blame/history is preserved.

Not run: the repo's full lint/typecheck (a full npm ci does not complete from this network). The change is docs-only and confined to docs/; no code or test is touched.

AI use

• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code — audited both documents against current code, implemented the archive move and banner, and authored the commit. The commit carries a Generated-by: Claude (Claude Code) trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

(Both unchecked: this is a docs-only change with no test surface; the relevant source-legal-inventory test passes as noted under Verification.)

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No