Shorten the README; move the technical detail into docs/

The README keeps what the tool is, how to install it and the first commands,
and points to the guide on docs.ihasmail.org. Everything else moves, whole,
into docs/ and CONTRIBUTING.md, where it is organized for readers who want
the detail. Where the old README disagreed with the code, the code wins.

CONTRIBUTING.md

@@ -0,0 +1,51 @@
+# Contributing to stalwart-migrator
+
+How to build and test the tool, and where its design is written down. For what
+the tool does and how to use it, see the [README](README.md).
+
+## Build and test
+
+```sh
+go build ./...
+go test ./...
+```
+
+Requires Go 1.26 or newer. The tool is Go standard library only — no external
+dependencies — and nothing third-party is vendored.
+
+To try a command from a checkout without installing it:
+
+```sh
+sudo go run ./cmd/stalwart-migrate preflight
+```
+
+[docs/rehearsal.md](docs/rehearsal.md) explains why even read-only commands need
+write access to the checkpoint directory.
+
+## Why not a shell script
+
+Stalwart's 0.15 → 0.16 boundary is not a drop-in binary swap: settings move,
+and the data directory has to be migrated rather than merely copied. The
+failure mode that matters is a half-migrated mail store with no way back —
+which is why backup verification and checkpointing are the design center
+rather than conveniences bolted on afterwards — and why the tool refuses to
+cut over until you confirm you have a way back.
+
+## Where the design is written down
+
+[`ARCHITECTURE.md`](ARCHITECTURE.md) covers the design in full: §1 on why a
+thin wrapper is insufficient, §4 on the migration phases, §5 on the checkpoint
+state machine, §6 on the CLI surface, and §8 on what is still open. It was
+written before the implementation and records the reasoning; where it and the
+code disagree about current behavior — the flags shown in §6, for example —
+the code and `stalwart-migrate <command> -h` are right.
+
+[docs/status.md](docs/status.md) has the state of each command and package, and
+what has been tested against real servers.
+
+## Testing against something real
+
+Where a phase runs an external program, its tests drive a fake one. That is
+sound for logic and ordering, and it is not evidence about production. Before
+trusting a change to a migration path, run it on a clone of a real server — see
+[Rehearse on a clone first](docs/rehearsal.md#rehearse-on-a-clone-first).