refactor: move proto/ to vantage-shared

vantage.proto documents the hand-written types in shared/grpc/pb, and
nothing compiles it. Keeping it in a different repository from the Go
types it describes meant the one rule holding them together — add the
message to both in the same commit — could not be followed at all.

server's rebuild trigger loses proto/, which it only carried as
insurance against exactly that split.

.gitea/workflows/server-deploy.yml

@@ -80,11 +80,11 @@ jobs:
                   # to three images, and no longer a way to ship a service
                   # against a shared/ it was never built with.
                   #
-                  # proto/ stays in server's list as insurance: the hand-written
-                  # pb now lives in vantage-shared, but a proto change made here
-                  # in the same push should not depend on the pin being bumped
-                  # in that same push to trigger a rebuild.
-                  flag server    '^(server/|proto/|default_steps/|go\.work)'
+                  # proto/ is gone too. It documents the wire types, and it
+                  # moved to sit beside the hand-written pb it describes, so
+                  # that a message added to one is added to the other in the
+                  # same commit. Nothing here reads it.
+                  flag server    '^(server/|default_steps/|go\.work)'
 
                   # web/ uses its own directory as the build context, so
                   # nothing outside it can affect it. It is the only front end