Display git commit info in the /about section. (#567)

This change detects the most recent git commit short hash and writes it to the `GIT_COMMIT_INFO` constant in `packages/cli/src/generated/git-commit.sh`, optionally appending the string "(local modifications)" if additional local changes after that commit are detected.

If set, this string is displayed in the `/about` dialog as well as passed into the `/bug` template.

Example:

```
> /about

╭───────────────────────────────────────────────────────────────────────────╮
│                                                                           │
│ About Gemini CLI                                                          │
│                                                                           │
│ CLI Version               development                                     │
│ Git Commit                43370ab (local modifications)                   │
│ Model                     gemini-2.5-pro-preview-05-06                    │
│ Sandbox                   sandbox-exec (minimal)                          │
│ OS                        darwin v23.11.0                                 │
│                                                                           │
╰───────────────────────────────────────────────────────────────────────────╯
```

Additionally, this change updates `.gitignore` to ignore the generated files, `scripts/clean.sh` to remove them, and adds a `npm run generate` stage for this and any other generators we need to write.

packages/cli/src/ui/hooks/slashCommandProcessor.test.ts

@@ -31,6 +31,7 @@ import { MessageType } from '../types.js';
 import { type Config } from '@gemini-code/server';
 
 import * as ShowMemoryCommandModule from './useShowMemoryCommand.js';
+import { GIT_COMMIT_INFO } from '../../generated/git-commit.js';
 
 vi.mock('./useShowMemoryCommand.js', () => ({
   SHOW_MEMORY_COMMAND_NAME: '/memory show',
@@ -244,6 +245,7 @@ Add any other context about the problem here.
 
 ## Diagnostic Information
 *   **CLI Version:** ${cliVersion}
+*   **Git Commit:** ${GIT_COMMIT_INFO}
 *   **Operating System:** ${osVersion}
 *   **Sandbox Environment:** ${sandboxEnvStr}
 *   **Model Version:** ${modelVersion}