Add option to disable sending usage telemetry

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

As a user, I want to be able to control what information I share with third parties.

### Proposed solution

Add a boolean `enableTelemetry` flag to `settings.json` to disable sending usage telemetry. This mirrors the existing flag in the Jetbrains GitHub Copilot plugin.

While I believe all such telemetry must be explicitly opt-in by default, at least adding the option would be a great first start, even if you don't expose it in the UI.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by locating how settings.json is parsed and where the CLI sends usage telemetry. Trace the existing settings flow and telemetry entry point, then verify that an enableTelemetry setting can control sending without changing the documented default behavior; the issue does not name a test file or implementation path.

Written by the indexing model from the issue text.