docs(docs): add AI harness readiness guidance [NT-4002]

[Johannes Maximilian Toball (jmtoball)]

Summary

• complete the public AI harness documentation set with repository-specific architecture and decision context
• preserve the existing public contributor and agent guidance
• add no internal catalog metadata or Agents Kit consumer wiring

Validation

• governance document thresholds and ADR discovery passed
• public-context redaction, referenced paths, and whitespace checks passed

Ticket: NT-4002

[Charles Hudson (phobetron)]

Looks good now!

One drawback with the global requirement that ADRs must be within docs, is that any repo that publishes GitHub Pages via a branch-based publishing flow might be affected, because it only supports publishing from / or /docs. Fortunately for us, we're using custom action-based publishing, so we don't have that limitation.