Added first-tier Grok support for the browser extension on grok.com.

What’s new:
- Added Grok as a supported chat surface.
- Added Grok to provider/model targeting alongside ChatGPT, Claude, and Gemini.
- Added Grok-aware prompt adaptation and continuity handoff support.
- Added support for reading, reviewing, and applying drafts on grok.com.
- Added narrow grok.com content-script matching without adding X/Twitter permissions.
- Added regression coverage for Grok surface detection, draft handling, manifest scope, provider routing, and review/apply flow.

Also improved:
- Updated supported-platform documentation and store submission notes.
- Tightened source-package exclusions for cleaner review uploads.
- Rebuilt Chrome, Firefox, and source packages for version 2.1.2.

Version 2.1.1 fixes a duplicate-output issue on ChatGPT pages by removing a legacy response overlay script that could append a flattened second copy of assistant responses.

Also included:
- Keeps ChatGPT response streaming untouched
- Preserves the normal Prompt Accelerator toolbar and review workflow
- No new permissions
- Packaging and Firefox AMO validation cleanup