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,
   },
 ];

src/root.ts

@@ -30,10 +30,10 @@ export function isServerAvailable(server: ServerConfig): boolean {
 }
 
 // Pick Server By File Extension - match[] entries are matched against the
-// file's extension (no dot). First available server in the registry wins.
+// file's extension (no dot). First available, non-diagnosticsOnly server wins.
 export function pickServer(filePath: string): ServerConfig {
   const ext = path.extname(filePath).replace(/^\./, "");
-  const hit = servers.find((s) => s.match.includes(ext) && isServerAvailable(s));
+  const hit = servers.find((s) => s.match.includes(ext) && !s.diagnosticsOnly && isServerAvailable(s));
   if (!hit) {
     throw new UnsupportedExtensionError(`.${ext}`);
   }

src/types.ts

@@ -34,6 +34,10 @@ export interface ServerConfig {
   // Idle TTL - Daemon keeps one server alive per (id, rootDir) and evicts
   // it after this many ms of inactivity. Defaults to 5 minutes.
   idleTtlMs?: number;
+  // Diagnostics Only - When true, this server is excluded from
+  // hover/definition/references/completion/documentSymbol but included
+  // in lsp_diagnostics and auto-check.
+  diagnosticsOnly?: boolean;
 }
 
 // Supported high-level commands exposed via the CLI. Extend this union