[codex] verify OpenClaw ACP detection

[NitayRabi]

Summary

• verify OpenClaw detection by proving acpx openclaw can initialize ACP, not just by finding an executable
• try each openclaw PATH candidate and persist the first working PATH ordering
• document the broken-wrapper failure mode and add regression coverage

Root Cause

OpenClaw detection accepted executable presence even when the first openclaw on PATH was a wrapper that failed before ACP initialize.

Validation

• go test ./internal/app ./internal/acpx
• go test ./...