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/template.md

@@ -0,0 +1,25 @@
+# NNNN. Title in the imperative, naming the decision
+
+- **Status:** Proposed | Accepted | Deprecated | Superseded by [NNNN](NNNN-slug.md)
+- **Area:** Architecture | Chain | Crypto | Net | SPV | Storage | Wallet | UI | Testing | Distribution | Scope | Process
+- **Recorded:** YYYY-MM-DD
+
+## Context
+
+The forces at play: the problem, the constraints, what was tried or rejected.
+Facts only; link to code, issues or specs instead of restating them.
+
+## Decision
+
+What we do, in the active voice ("We use…"). Specific enough that a reviewer
+can check the code against it.
+
+## Consequences
+
+What becomes easier, what becomes harder, new risks, follow-up work.
+Be honest about the downsides.
+
+## References
+
+- `path/to/code.cs`
+- Related ADRs, specs (BIPs), issues