docs: distribute package-specific GEMINI.md context to each package (#22734)

packages/vscode-ide-companion/GEMINI.md

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+# Gemini CLI VS Code Companion (`gemini-cli-vscode-ide-companion`)
+
+VS Code extension that pairs with Gemini CLI, providing direct IDE workspace
+access to the CLI agent.
+
+## Architecture
+
+- `src/extension.ts`: Extension activation and lifecycle.
+- `src/ide-server.ts`: Local server exposing IDE capabilities to the CLI.
+- `src/diff-manager.ts`: Diff viewing and application.
+- `src/open-files-manager.ts`: Tracks and exposes open editor files.
+- `src/utils/`: Shared utility functions.
+
+## Development
+
+- Requires VS Code `^1.99.0`.
+- Build: `npm run build` (uses esbuild).
+- Launch via VS Code's "Run Extension" debug configuration.
+
+## Testing
+
+- Run tests: `npm test -w gemini-cli-vscode-ide-companion`
+- Tests use standard Vitest patterns alongside VS Code test APIs.