ci: drop the :main tag — main builds publish only the immutable :<sha>

:latest (release-only) is the single production pointer; a :main moving
tag just duplicated it. main pushes still gate + build (the :<sha> image
is the rollback point), but no longer publish a :main alias. The tag was
new and unreferenced, so nothing depends on it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -1,15 +1,15 @@
 # CI runs first; build only proceeds if all checks pass.
 #
 # Push to dev:        typecheck + lint + test + build :dev + :<sha>
-# Push to main:       typecheck + lint + test + build :main + :<sha>
+# Push to main:       typecheck + lint + test + build :<sha>  (no moving tag)
 # Tag v* (release):   typecheck + lint + test + build :latest + :<version> + :<sha>
 #
-# Both dev and main are gated AND built: dev is the working line, main the
-# protected/integration line, each its own deployable image (:dev / :main).
-# Running CI again on the main merge commit is intentional — main is
-# validated and built in its own right. The v* release tag is the ONLY
-# trigger that publishes :latest (the production pointer) plus the
-# immutable :<version> tag.
+# Both dev and main are gated AND built. dev pushes move the :dev tag; main
+# pushes publish only the immutable :<sha> image — no :main tag, because
+# :latest (release-only) is the single production pointer and a :main alias
+# would just duplicate it. Running CI on the main merge commit is intentional:
+# main is validated and its :<sha> image is the rollback point. The v* release
+# tag is the ONLY trigger that publishes :latest plus the immutable :<version>.
 #
 # Successive pushes to the SAME ref supersede each other (see concurrency
 # below), so rapid pushes don't stack identical work; dev and main runs are
@@ -142,7 +142,7 @@ jobs:
   build:
     name: Build & push image
     needs: [typecheck, lint, test]
-    # Build on dev, main, and v* tag pushes. dev → :dev, main → :main,
+    # Build on dev, main, and v* tag pushes. dev → :dev, main → (sha only),
     # tag → :latest + :<version>; every build also gets an immutable :<sha>.
     if: github.ref == 'refs/heads/dev' || github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
     runs-on: python-ci
@@ -171,7 +171,8 @@ jobs:
               TAGS="$TAGS,${{ env.IMAGE }}:dev"
               ;;
             refs/heads/main)
-              TAGS="$TAGS,${{ env.IMAGE }}:main"
+              # main publishes only the immutable :<sha> image (set above) —
+              # no :main tag; :latest (release-only) is the production pointer.
               BUILD_VERSION="main"
               ;;
             refs/tags/*)