Automatic mode detection in v1 makes it impossible to use a stored prompt with @claude triggers

**Describe the bug**
I want to provide a default prompt AND trigger code reviews solely with `@claude` mentions. It would be great if the default prompt was only used when the `@claude` mention isn't followed by an explicit prompt, but if this isn't possible, ignoring the explicit prompts is good enough for me.

I don't understand why `prompt` and `@claude` were changed to be mutually exclusive.

See https://github.com/anthropics/claude-code-action/blob/main/docs/faq.md#how-does-automatic-mode-detection-work

**To Reproduce**
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**Expected behavior**
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**Workflow yml file**
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**API Provider**

[ ] Anthropic First-Party API (default)
[ ] AWS Bedrock
[ ] GCP Vertex

**Additional context**
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