cli: upgrade cannot detect npm installs on Windows (spawns .cmd shims without a shell) #57579

Summary

opencode upgrade / update on Windows always fails with "Could not detect the installation method" for npm-based installs, because install-method detection spawns npm/pnpm/yarn without a shell, and Windows cannot launch .cmd shims that way. --method npm bypasses detection, but the detection bug remains.

Environment

• opencode version: 2.1.1 (also reproduced on 2.0.32 and 2.1.0)
• OS: Microsoft Windows [Version 10.0.26200.9457] (Windows 11 25H2, x64)
• Terminal: xterm-256color via agent harness; also fails headless (opencode update --print-logs)
• Shell: C:\WINDOWS\system32\cmd.exe
• Install/channel: release, npm install -g @opencode/cli (global prefix under the user profile)
• Active plugins: none found in config

Reproduction

1. On Windows, npm install -g @opencode/cli.
2. Run opencode update (or opencode upgrade).
3. Detection tries npm/pnpm/bun/yarn/vp and reports "Could not detect the installation method."

Minimal proof the spawn mode is the cause (healthy minimal PATH where where npm succeeds):

• subprocess.run(['npm','--version'], shell=False) -> FileNotFoundError / WinError 2.
• subprocess.run('npm --version', shell=True) -> 11.12.1.

(bun, a real .exe, spawns fine but correctly reports "not installed via bun".)

Expected Behavior

Detection recognizes the global npm install and upgrades it without requiring --method.

Actual Behavior

Could not detect the installation method. Pass --method to choose how to upgrade OpenCode.

The debug log shows spawning process command=npm args=["list","-g","--depth=0","@opencode/cli"] failing, because npm resolves to npm.cmd, which CreateProcess cannot launch directly (it only resolves .exe).

Additional Context

• Consistently reproducible (2.0.32, 2.1.0, 2.1.1).
• The project's own postinstall.mjs already documents this: "Windows cannot spawn npm's .cmd shim directly, so reuse the npm CLI that runs this script" via [process.execPath, process.env.npm_execpath]. Detection needs the same treatment (shell on win32, or resolve npm.cmd through ComSpec).
• Workaround: opencode upgrade --method npm bypasses detection. Note the install step DOES use a shell (its failures surface cmd.exe's "not recognized" error), so --method works as long as the user's PATH is healthy; it fails only when PATH itself is broken (separate local issue on my machine: an over-long PATH).
• Suggested fix: spawn package managers with a shell on Windows in both the detection and install paths, or invoke <mgr>.cmd via ComSpec.