fix: surface actual LLM error in TUI instead of generic 'LLM call failed'

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Darth-Hidious

crates/agent/src/agent_loop.rs

@@ -412,6 +412,10 @@ pub async fn run_turn(
             .await
             .map_err(|e| {
                 tracing::error!(error = %e, "LLM call failed: {e:#}");
+                // Surface error details in the UI, not just "LLM call failed"
+                emit(AgentEvent::TextDelta {
+                    text: format!("Error: {e:#}\n"),
+                });
                 e
             })
             .context("LLM call failed")?;