refactor: make executable resolution async and centralize spawn

- Rename findExecutable → findExecutableSync, add async findExecutable
  that uses promisified execFile instead of execFileSync
- Simplify Windows PATH resolution: rely on inherited env from Explorer
  instead of shelling out to PowerShell for registry PATH entries
- Add spawnProcess() helper that centralizes Windows shell/quoting concerns
- Update all agent providers and consumers to use async executable resolution
- Add windowsHide: true across all Windows child process calls

packages/server/src/server/agent/providers/claude-agent-commands.real.e2e.test.ts

@@ -2,13 +2,13 @@ import { describe, expect, test } from "vitest";
 import pino from "pino";
 
 import type { AgentSlashCommand } from "../agent-sdk-types.js";
-import { isCommandAvailable } from "../../../utils/executable.js";
+import { isCommandAvailableSync } from "../../../utils/executable.js";
 import { ClaudeAgentClient } from "./claude-agent.js";
 
 // Real-Claude contract coverage: validates slash command shape from a live Claude CLI session.
 describe("claude agent commands contract (real)", () => {
   test("lists slash commands with the expected contract", async () => {
-    expect(isCommandAvailable("claude")).toBe(true);
+    expect(isCommandAvailableSync("claude")).toBe(true);
 
     const client = new ClaudeAgentClient({
       logger: pino({ level: "silent" }),