Introduce optional gitCommitInfo as app-wide metadata.

This change adds an optional `gitCommitInfo` string to the app state,
which, if set by the GET_COMMIT_INFO environment variable when the
application is launched, will be displayed in the /about section. 

The `start.sh` and `build_package.sh` scripts set `GET_COMMIT_INFO` to
the most recent git commit's short hash if known, appending the string
"(local modifications)" if additional changes are detected beyond that
point.

packages/cli/src/gemini.ts

@@ -68,12 +68,14 @@ async function main() {
     const readUpResult = await readPackageUp({ cwd: __dirname });
     const cliVersion =
       process.env.CLI_VERSION || readUpResult?.packageJson.version || 'unknown';
+    const gitCommitInfo = process.env.GIT_COMMIT_INFO || 'N/A';
 
     render(
       React.createElement(App, {
         config,
         settings,
         cliVersion,
+        gitCommitInfo,
         startupWarnings,
       }),
     );