Related to https://b.corp.google.com/issues/423605555 - I figured this might be a simpler solution to start with, while still also being useful on its own even if we do implement that.
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.
