refactor(daemon): require explicit serverId on all daemon ops

Move all server matching logic to the extension/CLI side. The daemon no
longer calls pickServer() — it receives an explicit serverId (or
serverIds[] for diagnostics) and uses it directly for cache lookup and
server spawning.

Key changes:
- request op requires serverId: string
- diagnostics op requires serverIds: string[] — daemon fans out in
  parallel via Promise.allSettled and returns grouped map
- formatDiagnostics() handles grouped results with per-server headers
  when multiple servers contribute (single-server omits header)
- CLI picks servers locally before calling daemon helpers
- New pickDiagnosticServers() in extension returns all available,
  non-disabled servers matching the file extension

This makes multi-server diagnostics (e.g., typescript-language-server +
oxlint) work naturally — the extension decides which servers to query,
the daemon just executes.

src/daemon.ts

@@ -6,8 +6,9 @@ import * as fs from "node:fs";
 import * as net from "node:net";
 import * as path from "node:path";
 import { LspClient } from "./client.ts";
-import { findRoot, pickServer, pathToUri } from "./root.ts";
+import { findRoot, pathToUri } from "./root.ts";
 import type { ServerConfig } from "./types.ts";
+import { servers } from "../server.ts";
 import {
   logPath,
   socketPath,
@@ -50,14 +51,25 @@ function log(...args: unknown[]) {
   );
 }
 
-// Get Or Create Entry - Looks up the cached client for a file, spawning a
-// fresh LspClient if needed. The returned entry is guaranteed to have its
-// `ready` promise resolved before the caller uses it.
+// Find Server By ID - Looks up a ServerConfig from the registry by ID.
+// Throws if the ID is not registered.
+function findServerById(serverId: string): ServerConfig {
+  const server = servers.find((s) => s.id === serverId);
+  if (!server) {
+    throw new Error(`Unknown server ID: "${serverId}". Registered: ${servers.map((s) => s.id).join(", ")}`);
+  }
+  return server;
+}
+
+// Get Or Create Entry - Looks up the cached client for a server+file,
+// spawning a fresh LspClient if needed. The returned entry is guaranteed
+// to have its `ready` promise resolved before the caller uses it.
 async function getOrCreateEntry(
   filePath: string,
+  serverId: string,
   launch: LaunchContext,
 ): Promise<ClientEntry> {
-  const server = pickServer(filePath);
+  const server = findServerById(serverId);
   const rootDir = findRoot(filePath, server.rootMarkers);
   const key = `${server.id}::${rootDir}`;
   const existing = entries.get(key);
@@ -161,7 +173,7 @@ async function handle(req: DaemonRequest): Promise<DaemonResponse> {
     switch (req.op) {
       case "request": {
         const filePath = path.resolve(req.file);
-        const entry = await getOrCreateEntry(filePath, req.launch);
+        const entry = await getOrCreateEntry(filePath, req.serverId, req.launch);
         const { uri } = await syncFile(entry, filePath);
         bumpIdle(entry);
         const result = await entry.client.sendRequest(
@@ -172,15 +184,27 @@ async function handle(req: DaemonRequest): Promise<DaemonResponse> {
       }
       case "diagnostics": {
         const filePath = path.resolve(req.file);
-        const entry = await getOrCreateEntry(filePath, req.launch);
-        const { uri, changed } = await syncFile(entry, filePath);
-        bumpIdle(entry);
-        if (changed) entry.client.clearDiagnostics(uri);
-        const result = await entry.client.waitForDiagnostics(
-          uri,
-          req.timeoutMs ?? 1500,
+        const timeoutMs = req.timeoutMs ?? 1500;
+
+        // Fan-Out - Run diagnostics against all requested servers in
+        // parallel. Individual failures are captured, not thrown.
+        const results: Record<string, unknown> = {};
+        const settled = await Promise.allSettled(
+          req.serverIds.map(async (serverId) => {
+            const entry = await getOrCreateEntry(filePath, serverId, req.launch);
+            const { uri, changed } = await syncFile(entry, filePath);
+            bumpIdle(entry);
+            if (changed) entry.client.clearDiagnostics(uri);
+            const diag = await entry.client.waitForDiagnostics(uri, timeoutMs);
+            return { serverId, diag };
+          }),
         );
-        return { id: req.id, ok: true, result };
+        for (const outcome of settled) {
+          if (outcome.status === "fulfilled") {
+            results[outcome.value.serverId] = outcome.value.diag;
+          }
+        }
+        return { id: req.id, ok: true, result: results };
       }
       case "status": {
         const result = {