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/0012-encrypted-wallet-file.md

@@ -0,0 +1,26 @@
+# 0012. Wallet file encrypted with AES-256-GCM and PBKDF2-HMAC-SHA512
+
+- **Status:** Accepted
+- **Area:** Storage
+- **Recorded:** 2026-11-03 (retroactive: reconstructed from code, comments and changelog)
+
+## Context
+
+The wallet file contains the seed and keys; theft of the file must not mean theft of funds.
+
+## Decision
+
+`EncryptedFile` wraps the whole wallet document in a self-describing JSON container (`plm-wallet-aesgcm-v1`: kdf, iterations, salt, nonce, ciphertext, tag). AES-256-GCM, key from PBKDF2-HMAC-SHA512 with 600,000 iterations, a fresh 16-byte salt and 12-byte nonce on every save. The iteration count is stored in the file, so it can be raised later, and is capped at 10,000,000 on read because it is attacker-controlled. A wrong password or tampering raises `WrongPasswordException`; any malformed container raises `InvalidDataException`, never a raw parser error. Writes are atomic (temp file + rename).
+
+## Consequences
+
+- Tampering is detected before decryption; parameter upgrades stay backward compatible.
+- The GCM tag cannot tell a wrong password from a corrupted file.
+- Users may opt out of encryption (`Save` with a null password), only with an explicit UI warning.
+- Changes to parameters or format must update `SECURITY.md` in the same commit.
+
+## References
+
+- `src/Core/Storage/EncryptedFile.cs`
+- `src/Core/Storage/WalletStore.cs`
+- `SECURITY.md`