docs(dates): record the true-instant date contract

- CONTEXT.md: date/time vocabulary (true instant, wall clock, camera
  offset, display zone, filename date class, ...)
- ADR-0001: dates are true instants; wall clocks are a display concern
- un-ignore docs/adr/ so decisions land in git

.gitignore

@@ -10,6 +10,7 @@ database/target
 /tmp
 /docs/*
 !/docs/agents/
+!/docs/adr/
 /specs
 
 # Default ignored files