Add ADR template and first two architecture decision records

Document the errors-as-values pattern (ADR-001) and domain events
as plain data objects (ADR-002) to capture the reasoning behind
these foundational design choices.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/adr/000-template.md

@@ -0,0 +1,20 @@
+# ADR-NNN: [Title]
+
+**Date**: YYYY-MM-DD
+**Status**: accepted | superseded | deprecated
+
+## Context
+
+What is the problem or situation that motivates this decision?
+
+## Decision
+
+What did we decide, and why?
+
+## Alternatives Considered
+
+What other approaches were evaluated?
+
+## Consequences
+
+What are the trade-offs — both positive and negative?