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/0029-persistent-android-signing-keystore.md

@@ -0,0 +1,24 @@
+# 0029. Persistent Android release keystore kept out of git
+
+- **Status:** Accepted
+- **Area:** Distribution
+- **Recorded:** 2026-11-03 (retroactive: reconstructed from code, comments and changelog)
+
+## Context
+
+Android refuses to update an installed app signed with a different key, and the user would have to uninstall, which deletes the wallet data.
+
+## Decision
+
+Release APKs are signed with one persistent keystore generated once by `docker/keystore/generate-keystore.sh`. `release.keystore` is ignored by git; passwords are supplied through environment variables or prompts at each build and are never stored. `docker/build.sh android` refuses to run without the keystore.
+
+## Consequences
+
+- In-place updates keep working across releases.
+- Losing the keystore or its passwords permanently breaks in-place updates; it must be backed up outside the repository.
+
+## References
+
+- `docker/keystore/generate-keystore.sh`
+- `docker/keystore/README.md`
+- `.gitignore`