docs: add pets module plan

AGENTS.md

@@ -76,6 +76,8 @@ src/
 
 **Rule for contributors (including future Codex and Claude Code sessions):** if a feature requires a change to `_core` to support a new entity type, that's a smell — extend the registry instead.
 
+**Completion rule for module additions and behavior changes:** include the authenticated v1 API and OpenAPI documentation, assistant tools backed by that API, and a complete reminder lifecycle for every user-visible due date. API, agent, and UI must call the same scoped server services; create/update must synchronize unfired reminders and delete must cancel them.
+
 ---
 
 ## Data model (v1)

CLAUDE.md

@@ -77,6 +77,8 @@ src/
 
 **Rule for contributors (including future Codex and Claude Code sessions):** if a feature requires a change to `_core` to support a new entity type, that's a smell — extend the registry instead.
 
+**Completion rule for module additions and behavior changes:** include the authenticated v1 API and OpenAPI documentation, assistant tools backed by that API, and a complete reminder lifecycle for every user-visible due date. API, agent, and UI must call the same scoped server services; create/update must synchronize unfired reminders and delete must cancel them.
+
 ---
 
 ## Data model (v1)