fix(templates): add missing Assumptions section to spec template (#1939)

Author: ismaelJimenez

templates/spec-template.md

@@ -113,3 +113,16 @@
 - **SC-002**: [Measurable metric, e.g., "System handles 1000 concurrent users without degradation"]
 - **SC-003**: [User satisfaction metric, e.g., "90% of users successfully complete primary task on first attempt"]
 - **SC-004**: [Business metric, e.g., "Reduce support tickets related to [X] by 50%"]
+
+## Assumptions
+
+<!--
+  ACTION REQUIRED: The content in this section represents placeholders.
+  Fill them out with the right assumptions based on reasonable defaults
+  chosen when the feature description did not specify certain details.
+-->
+
+- [Assumption about target users, e.g., "Users have stable internet connectivity"]
+- [Assumption about scope boundaries, e.g., "Mobile support is out of scope for v1"]
+- [Assumption about data/environment, e.g., "Existing authentication system will be reused"]
+- [Dependency on existing system/service, e.g., "Requires access to the existing user profile API"]