…w sync (#13666)

## Summary

Fixes #13665

- **Preserve user modifications** in OpenClaw config across restarts.
Previously `syncProviderConfig()` fully overwrote the model config on
every sync, discarding user changes (vision, custom context window,
etc.). Now uses a merge strategy: user-modified fields are preserved via
object spread, Cherry Studio-managed fields (`id`, `name`) always take
precedence.
- **Remove hardcoded `MODEL_CONTEXT_WINDOWS` map** — context window
defaults are delegated to OpenClaw's built-in `MODEL_CONTEXT_WINDOWS` in
`opencode-zen-models.ts`. Cherry Studio only provides `128000` as
fallback for new/unknown models on first sync.
- **Extract `OpenClawModelConfig` interface** with index signature to
support arbitrary user-added fields without type errors.

### Before
```typescript
models: provider.models.map((m) => ({
  id: m.id,
  name: m.name,
  contextWindow: 128000  // hardcoded for ALL models, overwrites every restart
}))
```

### After
```typescript
const existing = existingModelMap.get(m.id)
return {
  ...existing,           // preserve user's OpenClaw modifications
  id: m.id,              // Cherry Studio managed fields always win
  name: m.name,
  contextWindow: existing?.contextWindow ?? 128000  // OpenClaw handles model-specific values
}
```

## Test plan

- [x] `pnpm test:main` — 514 passed (including 44 OpenClawService tests)
- [x] `pnpm typecheck` — node + web both pass
- [x] `pnpm format` — no issues
- [ ] Manual: sync provider to OpenClaw → modify vision/context in
OpenClaw → restart → verify settings preserved

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Siin Xu <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: suyao <[email protected]>