release: 2.6.0: audit stage 5 - image and Postfix hardening

Cut without asking, as the owner said on 2026-10-15.

The supervisord socket is root:supervisor, the panel the group's one member
(M17). Postfix takes TLS 1.2 at least, the client too where TLS is
required; VRFY off; the banner without the distribution; the start refuses
a rate, size or milter timeout that is not a number, under its minimum or
past an int (L31). OpenDKIM logs through the syslog relay to the container
log and oversigns From: (L30). crashexit takes the container down after
five unexpected exits of a mail-path program in ten minutes (L32).
logrotate keeps its state under /data/log. extract-cert.sh refuses what is
not PEM and renames whole files (M21). Plus the findings of both reviews
(a39eda5). Commits dab7715, a39eda5, b09e525.

Rolled out to the test server as 2.5.4-post.95af363 (the same code): the
container healthy, every program RUNNING, syslog-relay among them;
OpenDKIM's start line in docker logs under "syslog: "; the postconf values
as set; /run/supervisor.sock root:supervisor 0770 and the panel reaches it;
/data/log/.logrotate.status written. No migration, schema 11.

CHANGELOG cut; pin 2.6.0 in deploy/docker-compose.yml, README, guide (and
its sample restore message); development.md "from pin" and the stage's row;
roadmap row and recommended order; stage 5 marked shipped in the plan. No
tag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,17 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+## [2.6.0] - 2026-10-15
+
+Stage 5 of the 2026-10-12 audit: the image and Postfix. The supervisor
+socket is the panel's alone; Postfix takes TLS 1.2 at least, and the start
+refuses settings that would kill its daemons at the first connection;
+OpenDKIM's log reaches the container log and it oversigns From:; a program
+that keeps crashing takes the container down; logrotate remembers what it
+rotated; extract-cert.sh never replaces a certificate with nothing. A new
+program, `syslog-relay`, and a new group, `supervisor`, in the image; no
+migration.
+
 ### Security
 
 - **Postfix and OpenDKIM could stop every program in the container.** The