Fix chat template quit handling

[pragnyanramtha]

Summary

• Check exit / quit chat commands before applying a prompt template
• Prevent templates with a prompt: value from turning quit into a model prompt
• Add a regression covering llm chat -t with a template prompt

Fixes #1336.

Validation

• uv run pytest tests/test_chat_templates.py -q
• uv run pytest tests/test_chat.py tests/test_chat_templates.py -q
• uv run ruff check llm/cli.py tests/test_chat_templates.py
• uv run black --check --target-version py313 llm/cli.py tests/test_chat_templates.py
• git diff --check

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