story-kit: instruct coder agents to find root causes for bugs (story 252)

Add "Bug Workflow: Root Cause First" guidance to all coder agent prompts
and system prompts. Adds a test ensuring all coder-stage agents include
root cause, git bisect/log, and anti-workaround instructions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/config.rs

@@ -632,6 +632,48 @@ name = "coder"
         assert_eq!(config.watcher, WatcherConfig::default());
     }
 
+    #[test]
+    fn coder_agents_have_root_cause_guidance() {
+        // Load the actual project.toml and verify all coder-stage agents
+        // include root cause investigation guidance for bugs.
+        let manifest_dir = std::path::Path::new(env!("CARGO_MANIFEST_DIR"));
+        let project_root = manifest_dir.parent().unwrap();
+        let config = ProjectConfig::load(project_root).unwrap();
+
+        let coder_agents: Vec<_> = config
+            .agent
+            .iter()
+            .filter(|a| a.stage.as_deref() == Some("coder"))
+            .collect();
+
+        assert!(
+            !coder_agents.is_empty(),
+            "Expected at least one coder-stage agent in project.toml"
+        );
+
+        for agent in coder_agents {
+            let prompt = &agent.prompt;
+            let system_prompt = agent.system_prompt.as_deref().unwrap_or("");
+            let combined = format!("{prompt} {system_prompt}");
+
+            assert!(
+                combined.contains("root cause"),
+                "Coder agent '{}' must mention 'root cause' in prompt or system_prompt",
+                agent.name
+            );
+            assert!(
+                combined.contains("git bisect") || combined.contains("git log"),
+                "Coder agent '{}' must mention 'git bisect' or 'git log' for bug investigation",
+                agent.name
+            );
+            assert!(
+                combined.to_lowercase().contains("do not") || combined.contains("surgical"),
+                "Coder agent '{}' must discourage adding abstractions/workarounds",
+                agent.name
+            );
+        }
+    }
+
     #[test]
     fn watcher_config_preserved_in_legacy_format() {
         let toml_str = r#"