docs: add private-repo runner parity guide

[jmcte]

Summary

• add a dedicated private-repo parity and runner-routing guide
• document which job classes fit Synology shell-only runners versus GitHub-hosted fallback
• call out the current parity gaps and point Docker-capable Linux work to issue Add an ephemeral Linux Docker-capable runner plane for private-repo parity #34

Testing

• corepack pnpm test
• corepack pnpm lint

Closes #35

[jmcte]

Added the private-repo parity and routing guide as a focused docs PR off current main. Local verification on this branch:\n\n- corepack pnpm test\n- corepack pnpm lint\n\nThis is intended to close #35 independently from the doctor/status implementation threads.

[ares-omt]

Validation pass looks clean. The new routing guide is consistent with the current shell-only Synology contract, keeps Docker-capable/private-repo parity gaps explicit, and points incompatible workloads at the tracked follow-up issue instead of quietly weakening the boundary. CI is green and I did not find a meaningful regression or trust-boundary problem in the docs change.