`tag.yml` is missing `on: workflow_call` trigger

Hello,

the `tag.yml` workflow is referenced in the [upjet provider template](https://github.com/crossplane/upjet-provider-template) as a reusable workflow:

```
jobs:
tag:
uses: crossplane-contrib/provider-workflows/.github/workflows/tag.yml@main
```

However, `tag.yml` currently only defines `on: workflow_dispatch`, not `on: workflow_call`. This means it cannot be called from another workflow and fails with:

```
failed to parse workflow: error parsing called workflow "" ->
"crossplane-contrib/provider-workflows/.github/workflows/tag.yml@main":
workflow is not reusable as it is missing a `on.workflow_call` trigger
```

Expected behavior: `tag.yml` should define `on: workflow_call` with the required inputs (version, message) so it can be used as a reusable workflow by providers generated from the upjet template.

Actual behavior: `tag.yml` only defines `on: workflow_dispatch`, causing any provider that delegates to it via uses: to fail immediately.

Suggested fix: Add a workflow_call trigger alongside the existing workflow_dispatch:

```
on:
workflow_dispatch:
inputs:
version:
description: 'Release version (e.g. v0.1.0)'
required: true
message:
description: 'Tag message'
required: true
workflow_call:
inputs:
version:
required: true
type: string
message:
required: true
type: string
```

Open .github/workflows/tag.yml and compare its existing workflow_dispatch trigger with the reusable-workflow usage shown in the issue. Check the required version and message inputs against the upjet provider template, then verify that providers can reference the workflow through uses: without the reusable-workflow parsing error.

Written by the indexing model from the issue text.