fix: remove stale tests that hang or assert dead behaviour

- Remove tool_merge_agent_work_returns_started and
  tool_get_merge_status_returns_running: these tested the old
  non-blocking API but tool_merge_agent_work now blocks in a poll
  loop, causing the tests to hang forever.

- Update coder_agents_have_root_cause_guidance: prompt no longer
  requires "git bisect" — check for bug workflow guidance instead.

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

server/src/config.rs

@@ -864,17 +864,12 @@ name = "coder"
             let combined = format!("{prompt} {system_prompt}");
 
             assert!(
-                combined.contains("root cause"),
-                "Coder agent '{}' must mention 'root cause' in prompt or system_prompt",
+                combined.contains("Bug Workflow") || combined.contains("trust the story"),
+                "Coder agent '{}' must include bug workflow guidance 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"),
+                combined.contains("surgical") || combined.to_lowercase().contains("minimal"),
                 "Coder agent '{}' must discourage adding abstractions/workarounds",
                 agent.name
             );