feat #14: Implement design guide (#15)

Reviewed-on: #15
Co-authored-by: phg <mail@philip-henning.com>
Co-committed-by: phg <mail@philip-henning.com>

CONTRIBUTING.md

@@ -41,6 +41,9 @@ Core code that is not reachable through the TUI is not yet a supported product
 feature. Do not advertise it in user-facing documentation until users can
 actually invoke it.
 
+Read the [TUI design guide](docs/design-guide.md) before changing layouts,
+components, interaction states, user-visible copy, or terminal-size behavior.
+
 ## Domain language and decisions
 
 Read [CONTEXT.md](CONTEXT.md) before naming user-visible concepts. Use its
@@ -111,6 +114,8 @@ Documentation is part of the behavior contract:
   persistence, recovery, configuration, or troubleshooting change.
 - Update [CONTEXT.md](CONTEXT.md) when the canonical domain language changes;
   keep it a glossary rather than an architecture guide or progress log.
+- Update the [TUI design guide](docs/design-guide.md) when an intentional UI
+  pattern should become the new project-wide standard.
 - Update ADR 0002 and the README if serialization stops being normalized or
   becomes byte-preserving.