fix(server): resolve gh executable via login shell for desktop users

Electron apps on macOS inherit a minimal PATH (/usr/bin:/bin:/usr/sbin:/sbin)
that doesn't include Homebrew or other user-installed tools. This caused
GitHub features (PR status, Create PR) to show as unavailable in the
desktop app even when gh CLI was installed and authenticated.

Extract findExecutable, resolveShellEnv, and related utilities from
provider-launch-config.ts into a shared utils/executable.ts module.
Use findExecutable("gh") with lazy caching so the login-shell resolution
happens once per daemon lifetime. Pass resolveShellEnv() as the process
env for all gh subprocess calls.

Update all consumers to import directly from utils/executable.ts —
no re-exports through provider-launch-config.ts.

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

@@ -1,8 +1,8 @@
 import { describe, expect, test } from "vitest";
 import pino from "pino";
 
-import { isCommandAvailable } from "../provider-launch-config.js";
 import type { AgentSlashCommand } from "../agent-sdk-types.js";
+import { isCommandAvailable } from "../../../utils/executable.js";
 import { ClaudeAgentClient } from "./claude-agent.js";
 
 // Real-Claude contract coverage: validates slash command shape from a live Claude CLI session.