Fix duplicate marketplace plugin loading (#364)

Reproduction:
- Enable `frontend-design@claude-code-plugins`
- Enable `frontend-design@claude-plugins-official`
- Start OpenClaude with both marketplace plugins active
- Both plugins load, but downstream command and skill scopes key off the short plugin name, so both collapse to `frontend-design` and can interfere with interactive startup

Fix:
- Collapse duplicate marketplace plugins by short name during merge
- Keep the enabled copy when enabled state differs; otherwise keep the later config entry
- Add regression coverage for both cases

src/utils/plugins/pluginLoader.test.ts

@@ -0,0 +1,71 @@
+import { describe, expect, test } from 'bun:test'
+
+import type { LoadedPlugin } from '../../types/plugin.js'
+import { mergePluginSources } from './pluginLoader.js'
+
+function marketplacePlugin(
+  name: string,
+  marketplace: string,
+  enabled: boolean,
+): LoadedPlugin {
+  const pluginId = `${name}@${marketplace}`
+  return {
+    name,
+    manifest: { name } as LoadedPlugin['manifest'],
+    path: `/tmp/${pluginId}`,
+    source: pluginId,
+    repository: pluginId,
+    enabled,
+  }
+}
+
+describe('mergePluginSources', () => {
+  test('keeps the enabled copy when duplicate marketplace plugins disagree on enabled state', () => {
+    const enabledOfficial = marketplacePlugin(
+      'frontend-design',
+      'claude-plugins-official',
+      true,
+    )
+    const disabledLegacy = marketplacePlugin(
+      'frontend-design',
+      'claude-code-plugins',
+      false,
+    )
+
+    const result = mergePluginSources({
+      session: [],
+      marketplace: [disabledLegacy, enabledOfficial],
+      builtin: [],
+    })
+
+    expect(result.plugins).toEqual([enabledOfficial])
+    expect(result.errors).toEqual([])
+  })
+
+  test('keeps the later copy when duplicate marketplace plugins are both enabled', () => {
+    const legacy = marketplacePlugin(
+      'frontend-design',
+      'claude-code-plugins',
+      true,
+    )
+    const official = marketplacePlugin(
+      'frontend-design',
+      'claude-plugins-official',
+      true,
+    )
+
+    const result = mergePluginSources({
+      session: [],
+      marketplace: [legacy, official],
+      builtin: [],
+    })
+
+    expect(result.plugins).toEqual([official])
+    expect(result.errors).toHaveLength(1)
+    expect(result.errors[0]).toMatchObject({
+      type: 'generic-error',
+      source: legacy.source,
+      plugin: legacy.name,
+    })
+  })
+})