Drop unused yarn handling from command runner

No emitter target uses yarn; the presets and docs are all npm/pnpm. Remove yarn from the package-manager passthrough-separator check and the Windows shell-shim regex.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: l0lawrence

eng/emitter-diff/src/cli.ts

@@ -132,7 +132,7 @@ function tokenizeCommand(command: string): string[] {
 /**
  * Build the full argv (command args + passthrough) for a regenerate command.
  *
- * `npm`/`pnpm`/`yarn run <script>` swallow subsequent flags as their own config
+ * `npm`/`pnpm run <script>` swallow subsequent flags as their own config
  * unless a `--` separator forwards them to the script (e.g. `npm run regenerate
  * -- --name foo`). When the command is such a package-manager `run` invocation
  * and the user supplied passthrough args, insert that `--` automatically so
@@ -143,7 +143,7 @@ function buildRegenerateArgs(commandArgv: string[], passthrough: string[]): stri
   if (passthrough.length === 0) return base;
 
   const bin = (commandArgv[0] ?? "").toLowerCase().replace(/\.(cmd|exe|ps1)$/, "");
-  const isPm = bin === "npm" || bin === "pnpm" || bin === "yarn";
+  const isPm = bin === "npm" || bin === "pnpm";
   // Locate the actual run subcommand (npm also spells it `run-script`).
   const runIndex = isPm ? base.findIndex((a) => a === "run" || a === "run-script") : -1;
   // A `--` only separates script args when it comes after that subcommand.

eng/emitter-diff/src/util.ts

@@ -65,10 +65,10 @@ export interface RunOptions {
  */
 export function run(cmd: string, args: string[], opts: RunOptions = {}): Promise<RunResult> {
   return new Promise((resolve, reject) => {
-    // Only route through a shell for Windows .cmd shims (npm/pnpm/npx/code/yarn).
+    // Only route through a shell for Windows .cmd shims (npm/pnpm/npx/code).
     // Native binaries like git/node are spawned directly to avoid the shell
     // argument-escaping deprecation and quoting pitfalls.
-    const needsShell = process.platform === "win32" && /^(npm|pnpm|npx|yarn|code)$/.test(cmd);
+    const needsShell = process.platform === "win32" && /^(npm|pnpm|npx|code)$/.test(cmd);
     const spawnOpts: SpawnOptions = {
       cwd: opts.cwd,
       env: opts.env ?? process.env,