feat: Enable npx execution directly from GitHub URL

This commit modifies the packaging setup to allow the CLI to be
executed directly from its GitHub URL using `npx`, for example:
`npx https://github.com/google-gemini/gemini-cli` (once merged to main).
This is achieved without requiring the bundle to be checked into
the repository.

Key changes and motivations:

- Modify `scripts.prepare` to run `npm run bundle`:
  Ensures the CLI bundle is generated automatically when `npx`
  installs the package from a git URL. This replaces previous
  approaches (e.g., using `prepack`) which were not consistently
  triggered in the `npx` environment.

- Update `scripts.bundle` to use a direct path for `esbuild`
  and externalize `sqlite3`:
  Using `node_modules/.bin/esbuild` provides a more reliable way
  to invoke the bundler. Externalizing `sqlite3` is crucial for
  correctly handling its native addon, preventing runtime errors.

- Add `bin`, `files`, and root `sqlite3` dependency:
  - The `bin` field defines the `gemini` command.
  - The `files` array ensures the generated `bundle/` directory is
    recognized by npm.
  - `sqlite3` is added as a root dependency to ensure it's
    installed by `npx` when `gemini-code` is fetched, allowing the
    externalized module to be resolved.

These changes collectively ensure that the necessary build artifacts
are created on-the-fly during `npx` installation, providing a
seamless execution experience directly from the GitHub repository URL.

eslint.config.js

@@ -33,6 +33,7 @@ export default tseslint.config(
       'packages/cli/dist/**',
       'packages/server/dist/**',
       'eslint-rules/*',
+      'bundle/**',
     ],
   },
   eslint.configs.recommended,