feat(docs): add Architecture Decision Records (ADR) guidelines to documentation

Author: stefaniuk

.github/copilot-instructions.md

@@ -18,7 +18,28 @@ After any source code change:
 3. [ ] Repeat until both commands complete with zero errors and zero warnings
 4. [ ] Do this automatically without prompting
 
+## Documentation (Mandatory)
+
+When making architectural or significant technical decisions, document them as Architecture Decision Records (ADRs):
+
+**What requires an ADR:**
+
+- [ ] Architectural style choices (e.g. event-driven vs layered, monolith vs microservices)
+- [ ] Architectural pattern choices (e.g. composition over inheritance, repository pattern)
+- [ ] Language and framework selections
+- [ ] Any other significant technical decision that shapes the system
+
+**ADR requirements:**
+
+- [ ] Use the template at [docs/adr/adr-template.md](../docs/adr/adr-template.md)
+- [ ] Follow the existing ADR format for consistency
+- [ ] Always present 3 or more options with trade-offs
+- [ ] Include the conversational context that led to the decision
+- [ ] Document decisions regardless of whether you made them independently or were guided by the user
+
+This requirement is mandatory, especially during the spec-driven development cycle: `spec` → `plan` → `tasks` → `implement`.
+
 ---
 
-> **Version**: 1.2.1
-> **Last Amended**: 2026-01-13
+> **Version**: 1.3.0
+> **Last Amended**: 2026-01-18