feat: migrate builds plugin to go and track per-build records

Adds typed JSON build records under data/builds/<app>/<build-id>.{json,log} keyed on a stable base36 ULID-style DOKKU_BUILD_ID generated for every deploy. The new commands surface that history (builds:list, builds:info, builds:prune) and an operator-configurable retention via builds:set retention. The existing builds:cancel and builds:output now key on the build-id (with safe handling for already-finalized and abandoned records), and the per-build log file replaces journalctl as the durable source of truth for builds:output.

plugins/git/functions

@@ -142,7 +142,7 @@ git_receive_app() {
     local exit_code=0
     acquire_app_deploy_lock "$APP" "exclusive"
     git_build "$APP" "$REV" || exit_code="$?"
-    release_app_deploy_lock "$APP"
+    release_app_deploy_lock "$APP" "$exit_code"
     return "$exit_code"
   fi
 }