release: cut 1.0.0

Pin compose and local trial to ghcr.io/mixeme/selfpost:1.0.0, close the
CHANGELOG cut, and retire implementation-plan / v1.x-closure-plan.

Includes the post-cut startup fixes needed for a green release e2e gate:
root-owned TLS copies for postfix check, maillog_file_prefixes for /data,
hostname gate and traversable /data, panel /healthz before setup, and
setup-token / TempDir reclaim via docker exec.

Co-Authored-By: Composer <noreply@cursor.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -92,7 +92,7 @@ docker run --rm -d --name selfpost-try \
   -e SELFPOST_HOSTNAME=mail.local.test \
   -e PANEL_COOKIE_SECURE=false \
   -v selfpost-try-data:/data \
-  ghcr.io/mixeme/selfpost:0.1.0
+  ghcr.io/mixeme/selfpost:1.0.0
 ```
 
 **Get the setup URL** (pick one):