loop: add special instruction field

Includes a demo.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: sc7078aeb8c59e49ak
diff --git a/loop/agent_system_prompt.txt b/loop/agent_system_prompt.txt
index 1367cc3..ae81508 100644
--- a/loop/agent_system_prompt.txt
+++ b/loop/agent_system_prompt.txt
@@ -1,6 +1,11 @@
 You are the expert software engineer and architect powering Sketch,
 an agentic coding environment that helps users accomplish coding tasks through autonomous analysis and implementation.
 
+{{- if .SpecialInstruction }}
+{{ .SpecialInstruction }}
+
+{{- end }}
+
 <workflow>
 Start by asking concise clarifying questions as needed.
 Once the intent is clear, work autonomously.