docs: reference guides for stamdata, audit log, feature flags + document-feature skill

Three how-it-works/how-to-extend reference docs (docs/reference/), each with a
coupling section, indexed in docs/README.md. New document-feature skill so docs
ship in the same diff as the code.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/README.md

@@ -23,6 +23,9 @@ condensed, cross-linked curriculum.
 | [architecture/0002-user-groups-and-bounded-contexts.md](reference/architecture/0002-user-groups-and-bounded-contexts.md) | ADR — user groups as actors; identity vs authorization.                                                                           |
 | [architecture/0003-cibg-huisstijl.md](reference/architecture/0003-cibg-huisstijl.md)                                     | ADR — adopt CIBG Huisstijl (vendored Bootstrap 5.2) + the token bridge.                                                           |
 | [architecture/0004-stamdata-as-code.md](reference/architecture/0004-stamdata-as-code.md)                                 | ADR — business-tunable reference data as typed, compile-time-validated config (not a production DB).                              |
+| [stamdata.md](reference/stamdata.md)                                                                                     | How stamdata (config-as-code reference data) is laid out, how to add a table with zero UI code, and why coupling stays low.       |
+| [audit-log.md](reference/audit-log.md)                                                                                   | How the data-minimised authz/PII-reveal audit trail is built, how to audit a new action, and the one-producer-hub coupling.       |
+| [feature-flags.md](reference/feature-flags.md)                                                                           | How runtime feature flags work (catalog-as-code + runtime state), how to add one, and the hand-wired gating coupling to watch.    |
 | [fp-tea-atomic-design.md](reference/fp-tea-atomic-design.md)                                                             | Long-form learning guide: FP + The Elm Architecture + atomic design.                                                              |
 | [wcag-checklist.md](reference/wcag-checklist.md)                                                                         | Manual WCAG checks automation can't catch (tab order, focus traps, reflow).                                                       |
 | [ui-ux-audit.md](reference/ui-ux-audit.md)                                                                               | Early UI/UX audit against NL Design System (predates ADR-0003 — read in that light).                                              |