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/0027-server-probe-and-live-server-test.md

@@ -0,0 +1,24 @@
+# 0027. Server probe tool and opt-in live-server test
+
+- **Status:** Accepted
+- **Area:** Testing
+- **Recorded:** 2026-11-03 (retroactive: reconstructed from code, comments and changelog)
+
+## Context
+
+Servers differ (Fulcrum, ElectrumX, electrs), can be on the wrong chain, lag behind or serve invalid proofs. The fake server cannot reveal how a real deployment behaves.
+
+## Decision
+
+`tests/PalladiumWallet.ServerProbe` is a read-only CLI that runs black-box checks against real servers through the wallet's own `ElectrumClient`: handshake and TLS fingerprint, `server.features` (genesis and `sha256`), tip freshness, genesis header, header linkage, a Merkle proof of a recent coinbase against the header root, fees, peers, error handling, latency and a pipelined burst, plus a cross-server tip comparison. `LiveServerTests` runs it from `dotnet test` only when `PALLADIUM_TEST_SERVERS` is set, so the default run stays offline.
+
+## Consequences
+
+- Operators and developers can qualify a server before using it.
+- The live test depends on third-party availability and is therefore opt-in.
+- The probe reuses production client code, so a pass means the wallet can use the server.
+
+## References
+
+- `tests/PalladiumWallet.ServerProbe/Program.cs`
+- `tests/PalladiumWallet.Tests/Net/LiveServerTests.cs`