## The Issue

Running the DDEV add-on update checker requires remembering the command
`curl -fsSL https://ddev.com/s/addon-update-checker.sh | bash`, which is
impossible to recall without looking it up every time. On top of that,
when maintaining multiple add-ons, you have to `cd` into each directory
and run the command individually — there's no way to check all add-ons
at once and then feed the combined output to an AI to fix everything in
one go.

## How This PR Solves The Issue

Adds `ddev utility addon-update-checker` (alias: `ddev ut addon-update-checker`),
which fetches and runs the update checker script automatically:

- If the target directory contains `install.yaml`, the checker runs there.
- Otherwise it scans immediate subdirectories for `install.yaml` and runs
  the checker in each one — useful for workspaces with multiple add-ons.
- Each run is clearly labelled with the directory, live output, and exit
  code (green for success, red for failure).
- Falls back to the raw GitHub script URL if the primary URL is unavailable.
- Uses `util.FindBashPath()` for cross-platform bash discovery.
- Supports `-d`/`--dir` flag to target a specific directory or workspace.

## Manual Testing Instructions

Run in a single add-on directory:

```shell
cd /path/to/my-addon   # must contain install.yaml
ddev utility addon-update-checker
```

Run across a workspace of add-ons:

```shell
ddev utility addon-update-checker -d /path/to/my-addons-workspace
```

Verify that a directory without `install.yaml` fails with a clear error:

```shell
ddev utility addon-update-checker -d /tmp
```

## Automated Testing Overview

`TestUtilityAddonUpdateCheckerCmd` in `utility-addon-update-checker_test.go`
covers three cases:

- `SingleAddon`: directory with `install.yaml` — verifies the dir is
  reported and an exit code is printed.
- `MultipleAddons`: workspace with two subdirs each containing
  `install.yaml` — verifies both are checked and two exit codes appear.
- `NoInstallYaml`: empty directory — verifies the command fails with
  "No install.yaml found".

## Release/Deployment Notes

New `ddev utility addon-update-checker` subcommand. No breaking changes.
Documentation added to `commands.md` and `creating-add-ons.md`.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>