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/session/system.test.ts

@@ -0,0 +1,59 @@
+import { describe, expect, test } from "bun:test"
+import path from "path"
+import { Agent } from "../../src/agent/agent"
+import { Instance } from "../../src/project/instance"
+import { SystemPrompt } from "../../src/session/system"
+import { tmpdir } from "../fixture/fixture"
+
+describe("session.system", () => {
+  test("skills output is sorted by name and stable across calls", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+      init: async (dir) => {
+        for (const [name, description] of [
+          ["zeta-skill", "Zeta skill."],
+          ["alpha-skill", "Alpha skill."],
+          ["middle-skill", "Middle skill."],
+        ]) {
+          const skillDir = path.join(dir, ".opencode", "skill", name)
+          await Bun.write(
+            path.join(skillDir, "SKILL.md"),
+            `---
+name: ${name}
+description: ${description}
+---
+
+# ${name}
+`,
+          )
+        }
+      },
+    })
+
+    const home = process.env.OPENCODE_TEST_HOME
+    process.env.OPENCODE_TEST_HOME = tmp.path
+
+    try {
+      await Instance.provide({
+        directory: tmp.path,
+        fn: async () => {
+          const build = await Agent.get("build")
+          const first = await SystemPrompt.skills(build!)
+          const second = await SystemPrompt.skills(build!)
+
+          expect(first).toBe(second)
+
+          const alpha = first!.indexOf("<name>alpha-skill</name>")
+          const middle = first!.indexOf("<name>middle-skill</name>")
+          const zeta = first!.indexOf("<name>zeta-skill</name>")
+
+          expect(alpha).toBeGreaterThan(-1)
+          expect(middle).toBeGreaterThan(alpha)
+          expect(zeta).toBeGreaterThan(middle)
+        },
+      })
+    } finally {
+      process.env.OPENCODE_TEST_HOME = home
+    }
+  })
+})