fix: only call setDefaultAgentForSort when user explicitly sets default_agent

The previous code read params.config.default_agent which is always populated to
"Sisyphus - Ultraworker" by existing logic (lines 196-204) even when the user
never configured default_agent. This silently overrode any custom agent_order.

Now we gate on configuredDefaultAgent (the user's explicit value) so the sort
shim rank map is only mutated when the user actually set default_agent.

Add regression test: agent_order without default_agent must preserve order.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin-handlers/agent-config-handler.ts

@@ -400,9 +400,10 @@ export async function applyAgentConfig(params: {
     );
   }
 
-  const resolvedDefault = (params.config as { default_agent?: string }).default_agent;
-  if (resolvedDefault) {
-    setDefaultAgentForSort(resolvedDefault);
+  if (configuredDefaultAgent) {
+    setDefaultAgentForSort(
+      (params.config as { default_agent?: string }).default_agent ?? configuredDefaultAgent,
+    );
   }
 
   const agentResult = params.config.agent as Record<string, unknown>;

src/shared/agent-sort-shim.test.ts

@@ -265,4 +265,25 @@ describe("agent-sort-shim", () => {
       })
     })
   })
+
+  describe("#given agent_order configured without default_agent", () => {
+    describe("#when setAgentSortOrder sets a non-canonical order and setDefaultAgentForSort is NOT called", () => {
+      test("#then the custom agent_order is preserved without implicit override", () => {
+        // given
+        setAgentSortOrder(["hephaestus", "sisyphus", "prometheus", "atlas"])
+        // setDefaultAgentForSort is intentionally NOT called (user did not set default_agent)
+        const sisyphus = { name: "Sisyphus - Ultraworker" }
+        const hephaestus = { name: "Hephaestus - Deep Agent" }
+        const prometheus = { name: "Prometheus - Plan Builder" }
+        const atlas = { name: "Atlas - Plan Executor" }
+        const input = [atlas, sisyphus, prometheus, hephaestus]
+
+        // when
+        const result = input.toSorted((a, b) => a.name.localeCompare(b.name))
+
+        // then — Hephaestus must remain first per the user's agent_order
+        expect(result).toEqual([hephaestus, sisyphus, prometheus, atlas])
+      })
+    })
+  })
 })