docs(runtime): define safe recovery for PDF rejections

[seekskyworld]

Summary

Refs #3285

Records the current runtime decision for provider PDF rejections: Maka does not perform a PDF-free retry until a provider adapter supplies structured, request-level evidence that proves the rejection occurred before provider work or tool effects. The document now includes the exact parsed error shapes accepted by the current classifier and the source paths audited.

Verification

• git diff --check
• Source audit against packages/runtime/src/provider-error-classification.ts, packages/runtime/src/model-adapter.ts, and packages/core/src/attachments.ts
• Documentation-only change; no runtime behavior or test suite is changed.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex authored the decision document and reviewed the linked runtime contracts.

Checklist

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

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No