[Bug] ACP runtime backend reports 'unavailable' on Windows despite acpx working

Description

ACP runtime backend reports 'unavailable' when using sessions_spawn({ runtime: 'acp' }) on Windows, even though acpx CLI works perfectly when invoked directly.

Environment

• OS: Windows (via Tailscale)
• OpenClaw: 2026.2.26
• acpx: 0.1.13

Steps to Reproduce

1. Install acpx globally: npm install -g acpx@latest
2. Install acpx locally in plugin directory
3. Add config to openclaw.json (plugins.entries.acpx.enabled, acp config, discord threadBindings)
4. Restart gateway
5. Try: sessions_spawn({ runtime: 'acp', agentId: 'codex', task: 'print hello' })

Expected Behavior

ACP session should start and return results from Codex.

Actual Behavior

Returns: "ACP runtime backend is currently unavailable. Try again in a moment."

Debugging Done

• acpx --version works (0.1.13)
• acpx.cmd --help in plugin directory works (exit code 0)
• Plugin shows as 'loaded' in openclaw plugins list
• Version check passes
• No errors in gateway logs

Workaround

Use direct CLI: codex exec "your task" - works perfectly

Additional Notes

• Direct Codex CLI (codex exec) works great
• This might be a Windows-specific issue with how the gateway spawns processes vs interactive sessions