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.

docs/README.md

@@ -96,6 +96,7 @@ This documentation covers the installation, configuration, and usage of Dokku -
 - [Plugin Management](advanced-usage/plugin-management.md) - Install and manage plugins
 - [Registry Management](advanced-usage/registry-management.md) - Docker registry integration
 - [Repository Management](advanced-usage/repository-management.md) - Git repository settings
+- [Build Tracking](advanced-usage/builds.md) - Inspect, cancel, and manage builds
 - [Deployment Tasks](advanced-usage/deployment-tasks.md) - Pre/post deploy hooks
 - [Event Logs](advanced-usage/event-logs.md) - Dokku event history
 - [Backup and Recovery](advanced-usage/backup-recovery.md) - Data backup strategies