Before:
App start -> spawn gateway -> visible cmd window flashes

After:
App start -> spawn gateway (windowsHide) -> no visible window -> clean startup

## Security Impact (required)

- New permissions/capabilities? No  
- Secrets/tokens handling changed? No  
- New/changed network calls? No  
- Command/tool execution surface changed? No  
- Data access scope changed? No  

---

## Repro + Verification

### Environment

- OS: Arch Linux x86_64 
- Kernel: 6.19.11-arch1-1
- Runtime/container: Node v22 .22.2
- Model/provider: N/A  
- Integration/channel (if any): Gateway process  
- Relevant config (redacted): default setup  

### Steps

1. Launch OpenClaw on Windows  
2. Observe startup behavior  
3. Monitor for visible terminal windows  

### Expected

- No command prompt windows appear  
- Gateway runs fully in background  

### Actual (before fix)

- Command prompt windows flash repeatedly during startup  

---

## Evidence

- [x] Issue reproduced before fix (visible flicker)  
- [x] Verified no window spawn after fix  

---

## Human Verification (required)

- Verified scenarios:
  - Standard startup on Windows  
  - Multiple gateway respawns do not produce flicker  

- Edge cases checked:
  - Rapid restart / respawn scenarios  
  - Background execution stability  

- What you did **not** verify:
  - All Windows versions  
  - Non-standard terminal environments  

---

## Review Conversations

- [x] I replied to or resolved every bot review conversation I addressed in this PR.  
- [x] I left unresolved only the conversations that still need reviewer or maintainer judgment.  

---

## Compatibility / Migration

- Backward compatible? Yes  
- Config/env changes? No  
- Migration needed? No  

---

## Risks and Mitigations

- Risk:
  - Potential unintended suppression of console output visibility on Windows  

- Mitigation:
  - Logging remains unaffected (stdout/stderr still captured)  
  - Behavior aligns with expected background process execution

## Changed files

- `src/infra/process-respawn.ts` (modified, +1/-0)