Add user-facing screenshot consent gate in Settings

[adamcohenhillel]

Patch 0009-OpenPhone-add-opt-in-screenshot-payloads.patch calls its screenshot capture "opt-in", but the opt-in is per-request — set by the caller (include_screenshot flag). The user has no single Settings toggle to disable screen capture globally.

Fix

• Add a Settings toggle (default off? on? — decide) that gates all system-server screenshot capture regardless of caller request.
• Route the decision through openphone_agent's hash-chained audit log so screenshot events are attributable.
• Update docs/CAPABILITIES.md to describe the toggle.

Size: M