refactor(sandbox): remove runtime file projection

[iamjr15]

Summary

• remove the unconsumed /workspace/.cheatcode/runtime.json projection
• remove every best-effort provider write triggered by process start, restart, reconciliation, kill, status, and sandbox reset
• keep Durable Object process records as the single lifecycle authority
• remove documentation for the non-portable file

Why

Production QA proved Daytona's persistent-volume toolbox and process views do not provide a coherent root-level file projection for this path. The file has no runtime consumer and was explicitly non-authoritative, so retaining repeated best-effort writes would add provider calls and warnings without giving users a reliable artifact. A clean implementation keeps the authoritative Durable Object state only.

Verification

• pnpm turbo lint typecheck build — 76/76 tasks passed on Node 24.18.0
• affected architecture boundary check — passed, 103 modules / 194 dependencies
• affected Knip workspace — passed
• no remaining runtime.json or manifest-sync references
• git diff --check — passed
• exactly one Drizzle migration remains; no database changes