tui: fix cursor position calculation

The cursor index is into self.input, but the rendered buffer contains
the prompt prepended to the first line. Need to add prompt.len() to
get the correct character position when scanning the buffer.

scripts/provision-vllm.sh

@@ -51,7 +51,7 @@ exec vllm serve "$MODEL" \
     --max-model-len "$MAX_MODEL_LEN" \
     --gpu-memory-utilization "$GPU_MEMORY_UTILIZATION" \
     --enable-prefix-caching \
-    --tool-call-parser qwen3_xml \
+    --tool-call-parser hermes \
     --enable-auto-tool-choice \
     --reasoning-parser=qwen3 \
     --uvicorn-log-level warning