Allow configuring MCP slow-connection warning threshold

### Describe the feature or problem you'd like to solve

MCP servers that require authentication handshakes (e.g., OAuth, Entra ID) routinely take 5-10 seconds to connect. The CLI displays a warning for each server that exceeds the hardcoded 10-second threshold:

```
! MCP server 'server-a' is taking longer than expected to connect.
! MCP server 'server-b' is taking longer than expected to connect.
! MCP server 'server-c' is taking longer than expected to connect.
```

All servers connect successfully -- the warnings are purely cosmetic. With many MCP servers configured, startup is dominated by a wall of yellow warnings that adds noise without actionable information.

The `timeout` field in `mcp-config.json` controls the hard connection timeout (how long before giving up), but there is no way to control the warning threshold.

### Proposed solution

Add support for a `slowConnectionThresholdMs` field in `mcp-config.json`, either:

**Per-server** (preferred):
```json
{
"mcpServers": {
"my-server": {
"type": "stdio",
"command": "my-mcp-server",
"args": ["start"],
"timeout": 60000,
"slowConnectionThresholdMs": 30000
}
}
}
```

**Or globally:**
```json
{
"mcpServers": { "...": "..." },
"slowConnectionThresholdMs": 30000
}
```

The internal plumbing already supports this -- the slow-connection monitor class constructor accepts `slowConnectionThresholdMs` as an option, it's just not wired to any user-facing config.

### Example prompts or workflows

1. User configures multiple MCP servers that authenticate via OAuth or similar
2. Each server takes ~5-10s to complete auth + upstream connect
3. User sets `"slowConnectionThresholdMs": 30000` to suppress warnings for expected auth latency
4. Startup shows clean output with no false-positive warnings

### Additional context

- The slow-connection monitor class already accepts `slowConnectionThresholdMs` in its constructor options -- this is a config-wiring change, not a new feature
- The default 10s threshold is hardcoded as a constant
- This primarily affects users with authenticated MCP proxies (OAuth servers, enterprise auth flows)
- A single OAuth/auth handshake can take ~5s, so even one hop can approach the 10s threshold