fix(release): drop version image tags, mint the rollback unit on main

The image tag map was the inverse of family rules 145 and 147 on every
count: it published :vYYYY.MM.DD.HHMM that nobody pinned, published :main
that rule 147 says should not exist, and published no commit-addressable
image at all — so the rollback unit the rule names did not exist in this
repo. A bad main push had nothing to roll back to but the previous
release tag, which may be many commits back.

The whole map is now:

  dev  → :dev
  main → :latest + :<sha>
  tag  → :latest

A release refreshes the channel and mints nothing else. The tag build
rebuilds the SAME SOURCE as main's build minutes earlier, differing only
in which APK is baked in, so rule 145's immutability clause applies
directly: move the channel tag, never re-push a commit-addressable one.
:latest has to move here rather than waiting for the next main push, or
the channel would carry the previous release's APK indefinitely — a
channel that cannot refresh itself (rule 146).

Two consequences that are not optional:

The verify job asserted the :<version> image existed. With version tags
gone that would fail every release for a tag nothing mints. Re-pointed at
the :<sha> image rather than deleted — deleting it is the tempting way to
make a failing guard go green, and it earns its keep twice now: it still
catches an image push that silently did not happen, and it additionally
proves the ordering, since a tag cut on a commit whose main build never
completed has no rollback target.

The server's self-reported version was the literal string "main" or
"dev". That was survivable while :vYYYY.MM.DD.HHMM existed to identify a
build; with version tags gone it is the ONLY thing that says which build
is running, and two dev images months apart were indistinguishable. It
now carries the derived name from ci/version.sh on every lane, with the
channel as a sibling field (rule 149) rather than folded into the string.
Surfaced at /healthz and beside the version in Settings.

Guards added for each arm of the policy, and every one was falsified
against the specific regression it names before committing. That caught
two real bugs in the guards themselves: stepBody cut at the next
`- name:`, which returns an EMPTY body for the last step in a job and
made the assertions pass vacuously, and its replacement cut at any blank
line followed by indentation, which truncated a step mid-run-block. The
helper now refuses an empty body outright.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

Dockerfile

@@ -15,12 +15,21 @@ COPY . .
 # Overwrite the committed placeholder with the freshly-built SPA assets.
 COPY --from=web /web/build ./web/build
 ENV CGO_ENABLED=0
-# Version stamping: release.yml passes the git tag via MINSTREL_VERSION
-# build-arg; local `docker build` falls back to "dev". Surfaced at
-# /healthz for operator-side image-version verification.
+# Version stamping. release.yml passes the DERIVED version name
+# (YYYY.MM.DD.HHMM) and the lane's channel; a local `docker build` falls back
+# to "dev"/"local". Both are surfaced at /healthz.
+#
+# These are two values on purpose (family rule 149): the same commit built on
+# dev and on main reports the same NAME and differs only in CHANNEL. Folding
+# the channel into the version string is what the rule forbids — the version
+# used to BE the channel word here ("main"/"dev"), which meant two dev images
+# eight weeks apart were indistinguishable.
 ARG MINSTREL_VERSION=dev
+ARG MINSTREL_CHANNEL=local
 RUN go build -trimpath \
-    -ldflags="-s -w -X 'git.fabledsword.com/bvandeusen/minstrel/internal/server.ServerVersion=${MINSTREL_VERSION}'" \
+    -ldflags="-s -w \
+      -X 'git.fabledsword.com/bvandeusen/minstrel/internal/server.ServerVersion=${MINSTREL_VERSION}' \
+      -X 'git.fabledsword.com/bvandeusen/minstrel/internal/server.ServerChannel=${MINSTREL_CHANNEL}'" \
     -o /out/minstrel ./cmd/minstrel
 
 FROM debian:bookworm-slim