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/0019-data-location-resolution.md

@@ -0,0 +1,23 @@
+# 0019. Resolve the data root by explicit precedence
+
+- **Status:** Accepted
+- **Area:** Storage
+- **Recorded:** 2026-11-03 (retroactive: reconstructed from code, comments and changelog)
+
+## Context
+
+Desktop users want portable installs or a custom folder; Android must use its sandbox; existing installs must keep their data.
+
+## Decision
+
+`AppPaths.DataRoot()` resolves in order: `OverrideDataRoot` (set by the Android head to the app sandbox, or by tests), portable `palladium-data` folder next to the executable, user-chosen folder stored in a pointer file at a fixed bootstrap location, legacy location if it already holds data, then the platform default (`%APPDATA%\PalladiumWallet` on Windows, `~/.palladium-wallet` on Linux). Each network gets its own subfolder (config, wallets, certificates, servers).
+
+## Consequences
+
+- First-run wizard asks for a location only when none is determined; Android skips that step.
+- Static override seams complicate tests, which therefore share the xUnit collection `AppPaths`.
+
+## References
+
+- `src/Core/Storage/AppPaths.cs`
+- `src/App.Android`