feat(server): add diagnosticsOnly flag for lint-only servers

Add diagnosticsOnly?: boolean to ServerConfig. When set, the server is
excluded from pickServer() (hover/definition/references/completion/
documentSymbol) but still included in pickDiagnosticServers() for
lsp_diagnostics and auto-check.

Mark oxlint as diagnosticsOnly: true — it now contributes diagnostics
alongside typescript-language-server without interfering with navigation
or completion tools.

server.ts

@@ -64,5 +64,6 @@ export const servers: ServerConfig[] = [
     args: ["--lsp"],
     rootMarkers: [".oxlintrc.json", "oxlint.config.json"],
     languageId: "typescript",
+    diagnosticsOnly: true,
   },
 ];