[Bug]: Pattern system prompt merges user input into single system message, causing empty responses on OpenAI-compatible providers that require a user turn

### What happened?

When using a pattern with an OpenAI-compatible provider (e.g. Ollama Cloud via OPENAI_API_BASE_URL), Fabric sends only a single message with role=system containing both the pattern content and the user input appended at the # INPUT: section. No role=user message is sent. Several OpenAI-compatible providers require at least one user turn to generate a response and return an empty response when only a system message is present.

To reproduce

Configure Fabric with an OpenAI-compatible provider via OPENAI_API_BASE_URL (e.g. Ollama Cloud at https://ollama.com/v1)
Run any pattern: echo "test input" | fabric -p summarize
Observe empty response

Enabling --debug=4 confirms only one message is sent:

DEBUG: FABRIC->LLM request messages (1)
DEBUG: FABRIC->LLM [0] role=system content="# IDENTITY and PURPOSE\n...\n# INPUT:\n\nINPUT:\ntest input"
DEBUG: LLM->FABRIC response content=""

The same provider works correctly when called directly with separate system and user messages.

Root cause

In internal/core/chatter.go, BuildSession sets inputUsed = true whenever a pattern is loaded, then uses this flag to suppress appending the user message:

if len(request.Message.MultiContent) > 0 || (request.Message != nil && !inputUsed) {
session.Append(request.Message)
}

When inputUsed is true the user message is never appended, resulting in a single system message.

What I did to fix it for me but that might not be done because other providers

Remove the !inputUsed guard so the user message is always appended as a separate turn:

go
if len(request.Message.MultiContent) > 0 || request.Message != nil {
session.Append(request.Message)
}

And remove the now-unused inputUsed variable and assignment.

### Operating System

Linux - amd64

### OS Version

```shell
Debian 13
```

### How did you install Fabric?

Release Binary - Linux (amd64)

### Version

```text
Fabric version: v1.4.473
```

### Relevant log output

```shell

```

### Relevant screenshots (optional)

_No response_

Start in internal/core/chatter.go, where BuildSession decides whether to append the request message, and reproduce with an OpenAI-compatible provider such as Ollama Cloud using the documented fabric -p summarize command and debug output. Done means a pattern request sends separate system and user messages and no longer returns an empty response from providers that require a user turn.

Written by the indexing model from the issue text.