V1 [D3] Config reference discipline

[austin-barrington]

Context

Large HyperbytedbConfig risks env/TOML/docs drift (called out in engineering review notes).

Deliverables

• Rule in contributing/review rubric: new fields require TOML example, HYPERBYTEDB__ env doc, and user-guide row.
• Optional CI check or doc generation from config schema if feasible.

References

src/config.rs; docs/user-guide/configuration.md; docs/engineering/section-review-notes.md