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/daemonProtocol.ts

@@ -38,6 +38,7 @@ export type DaemonRequest =
       id: number;
       op: "request";
       file: string;
+      serverId: string;
       method: string;
       params: Record<string, unknown>;
       launch: LaunchContext;
@@ -46,6 +47,7 @@ export type DaemonRequest =
       id: number;
       op: "diagnostics";
       file: string;
+      serverIds: string[];
       timeoutMs?: number;
       launch: LaunchContext;
     }
@@ -57,6 +59,7 @@ export type DaemonRequestWithoutId =
   | {
       op: "request";
       file: string;
+      serverId: string;
       method: string;
       params: Record<string, unknown>;
       launch: LaunchContext;
@@ -64,6 +67,7 @@ export type DaemonRequestWithoutId =
   | {
       op: "diagnostics";
       file: string;
+      serverIds: string[];
       timeoutMs?: number;
       launch: LaunchContext;
     }