fix: stabilize agent and skill ordering in prompt descriptions (#18261)

Co-authored-by: Aiden Cline <aidenpcline@gmail.com>
Co-authored-by: Aiden Cline <63023139+rekram1-node@users.noreply.github.com>

packages/opencode/test/agent/agent.test.ts

@@ -384,6 +384,32 @@ test("multiple custom agents can be defined", async () => {
   })
 })
 
+test("Agent.list keeps the default agent first and sorts the rest by name", async () => {
+  await using tmp = await tmpdir({
+    config: {
+      default_agent: "plan",
+      agent: {
+        zebra: {
+          description: "Zebra",
+          mode: "subagent",
+        },
+        alpha: {
+          description: "Alpha",
+          mode: "subagent",
+        },
+      },
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const names = (await Agent.list()).map((a) => a.name)
+      expect(names[0]).toBe("plan")
+      expect(names.slice(1)).toEqual(names.slice(1).toSorted((a, b) => a.localeCompare(b)))
+    },
+  })
+})
+
 test("Agent.get returns undefined for non-existent agent", async () => {
   await using tmp = await tmpdir()
   await Instance.provide({