docs: fix false tool-call attribution in parser troubleshooting (#330)

The "failed to select primary docker image via llm call" symptom was
attributed to tool-call parser failures, but image selection is a plain
text completion (providers.go: prv.Call -> WrapGenerateFromSinglePrompt),
not a tool call. That error is a generic LLM-backend failure of the
`simple` agent type and is already covered in its own README section.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -1403,7 +1403,6 @@ Typical symptoms:
 - Backend or proxy errors such as `Failed to parse tool call arguments as JSON` (often surfaced through a LiteLLM proxy as an HTTP 500), or other unexpected 5xx/4xx responses from the LLM endpoint.
 - A flow that runs for a few steps and then stops responding to new input in the UI.
 - Repeated or looping tool calls that never converge.
-- A flow that fails right at the start with `failed to select primary docker image via llm call`, because the first action in a flow is an LLM tool call to choose the container image; a backend that cannot return a valid tool call fails at this step too.
 
 How to investigate: