docs: add docs-as-code process and one ADR per feature

docs/docs-as-code.md defines the workflow; docs/adr holds 31 ADRs
(retroactive, reconstructed from code and changelog) with a template and
index. docs/check-docs.sh validates ADR structure, index, cited code
paths, relative links and CLAUDE.md/AGENTS.md sync.

docs/adr/0003-core-without-ui-dependency.md

@@ -0,0 +1,24 @@
+# 0003. Layered architecture: Core knows nothing about the UI
+
+- **Status:** Accepted
+- **Area:** Architecture
+- **Recorded:** 2026-11-03 (retroactive: reconstructed from code, comments and changelog)
+
+## Context
+
+The same wallet logic must be driven by a GUI, a CLI and headless tests. If cryptography or network code leaked into view code it could not be tested without a display and could not be audited in isolation.
+
+## Decision
+
+Layers depend only downward: GUI → wallet domain → SPV/sync → network → cryptography → persistence. `src/Core` (`Chain/ Crypto/ Wallet/ Spv/ Net/ Storage/`) has no UI dependency. `src/App` and `src/Cli` depend only on `Core`; the UI reaches the network and cryptography exclusively through the wallet domain.
+
+## Consequences
+
+- `dotnet test` covers almost all behaviour headlessly.
+- The CLI is a second front end for free (0024).
+- Violations (e.g. a ViewModel opening a socket) are review blockers, not style issues.
+
+## References
+
+- `src/Core`
+- `src/Cli/Program.cs`