Rewrite the README for inbuxa-migrate

The README now says what the tool is in the suite's plain voice: the
archive in the middle, convergent reruns, --dry-run, the archive as a
backup, the sources it reads (Stalwart among the JMAP ones), the
INBUXA_MIGRATE_* credential variables, installing from the Gitea release
with SHA256SUMS, building and testing, and the license with the lineage
line. Upstream's logo, badges and install channels are gone.

The full command and flag reference moves to docs/usage.md, with the
renamed binary and variables. The CHANGELOG gains a section for this first
release, above upstream's history.

CHANGELOG.md

@@ -1,6 +1,26 @@
 # Change Log
 
-All notable changes to this project will be documented in this file. This project adheres to [Semantic Versioning](http://semver.org/).
+All notable changes to this project are recorded here. Versions are dates:
+release `v2026.9.30` is version `2026.9.30`.
+
+## [Unreleased] -- 2026.9.30
+
+### Changed
+- Renamed to inbuxa-migrate: the binary, the crate, and the credential
+  variables, now `INBUXA_MIGRATE_PASSWORD`, `INBUXA_MIGRATE_TOKEN`,
+  `INBUXA_MIGRATE_EWS_CLIENT_SECRET` and `INBUXA_MIGRATE_GRAPH_TOKEN`. The old
+  names are not read. Archives written by Vandelay are not read either.
+- Registry calls (`x:Account`, `x:Domain` and the other `x:` types) use the
+  capability the server advertises: `urn:inbuxa:jmap:registry` on inbuxa,
+  `urn:stalwart:jmap` on a Stalwart source. A server that advertises neither
+  gets a clear error instead of a request it never offered to accept.
+- Released as Linux archives for amd64 and arm64 on the Gitea release page,
+  with `SHA256SUMS`. The npm, Homebrew, shell and PowerShell installers and
+  the MSI are gone.
+
+---
+
+# Earlier releases (upstream Vandelay)
 
 ## [1.0.11] - 2026-09-28