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/0013-versioned-wallet-document-and-sync-cache.md

@@ -0,0 +1,24 @@
+# 0013. Versioned JSON wallet document with an offline sync cache
+
+- **Status:** Accepted
+- **Area:** Storage
+- **Recorded:** 2026-11-03 (retroactive: reconstructed from code, comments and changelog)
+
+## Context
+
+Wallet metadata (labels, contacts, gap limit) and the last synced state must persist across sessions and across app upgrades; balances should be viewable offline.
+
+## Decision
+
+`WalletDocument` is a versioned JSON schema (`CurrentVersion = 1`) with a migration hook on open. It holds the seed or keys (so it must be encrypted, 0012), account xpub and path, labels, contacts, gap limit (default 20) and a `SyncCache` with balance/history. Only confirmed transactions are cached because unconfirmed ones can change (RBF); only the fully verified end state of a sync is persisted (0008).
+
+## Consequences
+
+- Reopening a wallet is fast and works offline.
+- Schema changes require a version bump and a migration, tested before release.
+- A large cache makes saves heavy, so `WalletStore.SaveAsync` moves serialization and I/O off the UI thread.
+
+## References
+
+- `src/Core/Storage/WalletDocument.cs`
+- `src/Core/Storage/WalletStore.cs`