Function parse_opencode_output 

pub fn parse_opencode_output(
    stdout: &str,
) -> Result<(String, f64, u64), AppError>

Parse the NDJSON output from opencode run --format json.

The output has 3 event types:

• step_start: ignored
• text: .part.text contains the LLM response text
• step_finish: .part.tokens and .part.cost for accounting

Returns (response_text, cost, tokens).