feat: implement immutable form definitions

AGENTS.md

@@ -0,0 +1,24 @@
+# GovOPlaN Forms Codex Guide
+
+## Scope
+
+This repository owns immutable reusable form definitions and their designer.
+Runtime values, drafts, receipts, attachments, signatures, and handoffs belong
+to `govoplan-forms-runtime` or the corresponding domain owner.
+
+## Working Rules
+
+- Resolve cross-module behavior through Core contracts and capabilities; do not
+  import another optional module's tables or WebUI pages.
+- Keep definition revisions immutable and tenant-bound. Schema changes create a
+  new revision and use optimistic concurrency.
+- Keep submitted values out of Forms events, logs, and definition storage.
+- Every behavior or UI change must update the relevant user/admin documentation
+  and architecture declaration in the same change.
+
+## Verification
+
+```bash
+PYTHONPATH=src:/mnt/DATA/git/govoplan-core/src \
+  /mnt/DATA/git/govoplan/.venv/bin/python -m unittest discover -s tests
+```