docs(agents): add doc-current-state convention + sharpen the summary worked example (review)

Two review findings on the AGENTS.md additions:

- Add a convention to § Type Safety and Documentation: document the CURRENT
  state (what + why), never the PROCESS/HISTORY of how the code got there. No
  "previously/now/used-to/replaces/the old X" in comments or JSDoc — that rots
  on the next change and belongs in the commit message / PR / RFC. A standing
  contrast against a live alternative is fine; a contrast against the codebase's
  past is not.

- The "tests document behavior" worked example overstated the audit as "nothing
  in production read or wrote" the summary. The backends DID write it (JSONL
  sidecar, SQLite updated_at); what made it dead was no CONSUMER and no
  update() caller. Corrected so a future reader does not infer the write path
  never existed.