test(config): add regression coverage for legacy migration bugs

Lock the current legacy config and plugin migration failures in place before the fixes land so the three regressions stay covered.

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

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

src/shared/external-plugin-detector.test.ts

@@ -1,18 +1,26 @@
-import { describe, expect, test, beforeEach, afterEach } from "bun:test"
+import { afterEach, beforeEach, describe, expect, mock, test } from "bun:test"
 import { detectExternalNotificationPlugin, getNotificationConflictWarning, detectExternalSkillPlugin, getSkillPluginConflictWarning } from "./external-plugin-detector"
 import * as fs from "node:fs"
 import * as path from "node:path"
 import * as os from "node:os"
 
+async function importFreshExternalPluginDetectorModule(): Promise<typeof import("./external-plugin-detector")> {
+  return import(`./external-plugin-detector?test=${Date.now()}-${Math.random()}`)
+}
+
 describe("external-plugin-detector", () => {
   let tempDir: string
+  let tempHomeDir: string
 
   beforeEach(() => {
     tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "omo-test-"))
+    tempHomeDir = fs.mkdtempSync(path.join(os.tmpdir(), "omo-home-"))
   })
 
   afterEach(() => {
+    mock.restore()
     fs.rmSync(tempDir, { recursive: true, force: true })
+    fs.rmSync(tempHomeDir, { recursive: true, force: true })
   })
 
   describe("detectExternalNotificationPlugin", () => {
@@ -399,6 +407,31 @@ describe("external-plugin-detector", () => {
       expect(result.pluginName).toBe("opencode-skills")
     })
 
+    test("should detect user-level opencode-skills when project config exists without plugins", async () => {
+      // given
+      const projectConfigDir = path.join(tempDir, ".opencode")
+      const userConfigDir = path.join(tempHomeDir, ".config", "opencode")
+      fs.mkdirSync(projectConfigDir, { recursive: true })
+      fs.mkdirSync(userConfigDir, { recursive: true })
+      fs.writeFileSync(path.join(projectConfigDir, "opencode.json"), JSON.stringify({}))
+      fs.writeFileSync(path.join(userConfigDir, "opencode.json"), JSON.stringify({ plugin: ["opencode-skills"] }))
+
+      const nodeOs = await import("node:os")
+      mock.module("node:os", () => ({
+        ...nodeOs,
+        homedir: () => tempHomeDir,
+      }))
+      const { detectExternalSkillPlugin: detectExternalSkillPluginFresh } = await importFreshExternalPluginDetectorModule()
+
+      // when
+      const result = detectExternalSkillPluginFresh(tempDir)
+
+      // then
+      expect(result.detected).toBe(true)
+      expect(result.pluginName).toBe("opencode-skills")
+      expect(result.allPlugins).toEqual(["opencode-skills"])
+    })
+
     test("should NOT match opencode-skills-extra (suffix variation)", () => {
       // given - plugin with similar name but different suffix
       const opencodeDir = path.join(tempDir, ".opencode")