build: move releases to CI and trim the Makefile

Merging a pull request into main is now the whole release. A Gitea
Actions workflow derives the CalVer tag, builds the image and pushes
it with :latest, so nothing is built locally any more.

That made image/push/release redundant, and with them the .release-tag
state file and the main-branch guard — the workflow only runs on main,
which is protected, so the guard had nothing left to catch. The digest
verification went too: it guarded a `make -j` race between image and
push that cannot happen in a single CI job.

seed, icons and vendor ran a few times a year and are written out in
the README instead. Makefile: 151 lines to 71.

Docs referenced the removed targets in sixteen places, including a
CONTRIBUTING note claiming the branch check "has to be local".

assets/icon.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512"
      role="img" aria-label="Foodster">
-  <!-- Source for the home-screen PNGs; `make icons` rasterises it.
+  <!-- Source for the home-screen PNGs; the README says how to rasterise it.
        Unlike favicon.svg this one is opaque and fixed-colour: a home screen
        icon cannot be transparent and must not change with the system theme.
        The bowl sits inside the central 80% so Android can mask it to any