feat(11.1-01): add summer docs:build with the docsite generator core

- internal/docsite loads docs/ with strict site.yaml and frontmatter decoding
- goldmark GFM pipeline renders pages into an embedded html/template shell
- emits .html pages, .md siblings, llms.txt, llms-full.txt, search-index.json
- output guard refuses unmarked non-empty dirs and --out inside --src or root
- docs/index.md and docs/setup/installation.md; /site/ is gitignored

cmd/summer/main.go

@@ -46,6 +46,7 @@ func toolCommands() []bonfire.Command {
 		delegateQueueWorkCommand(),
 		delegateScheduleRunCommand(),
 		delegateCommand("queue:clear", "Clear pending queued jobs in the app binary"),
+		docsBuildCommand(),
 	}
 }