fix(test): isolate discovery tests from global env var contamination

The discovery.test.ts was using process.env.CLAUDE_PLUGINS_HOME to set
the plugins directory, but this global state could be affected by other
tests running in parallel, causing flaky failures with errors like:
  Expected: "oh-my-openagent"
  Received: "demo"

Changes:
- Added pluginsHomeOverride option to PluginLoaderOptions type
- Modified discoverInstalledPlugins to accept optional pluginsHomeOverride
- Modified loadInstalledPlugins to accept optional pluginsBaseDir
- Updated all 3 discovery tests to use pluginsHomeOverride instead of
  relying on global process.env.CLAUDE_PLUGINS_HOME

This makes the tests properly isolated and deterministic regardless of
test execution order or parallelization.

Fixes CI failure on dev branch.

src/features/claude-code-plugin-loader/discovery.test.ts

@@ -59,7 +59,7 @@ describe("discoverInstalledPlugins", () => {
     )
 
     //#when
-    const discovered = discoverInstalledPlugins()
+    const discovered = discoverInstalledPlugins({ pluginsHomeOverride: pluginsHome })
 
     //#then
     expect(discovered.errors).toHaveLength(0)
@@ -94,7 +94,7 @@ describe("discoverInstalledPlugins", () => {
     )
 
     //#when
-    const discovered = discoverInstalledPlugins()
+    const discovered = discoverInstalledPlugins({ pluginsHomeOverride: pluginsHome })
 
     //#then
     expect(discovered.errors).toHaveLength(0)
@@ -129,7 +129,7 @@ describe("discoverInstalledPlugins", () => {
     )
 
     //#when
-    const discovered = discoverInstalledPlugins()
+    const discovered = discoverInstalledPlugins({ pluginsHomeOverride: pluginsHome })
 
     //#then
     expect(discovered.errors).toHaveLength(0)

src/features/claude-code-plugin-loader/discovery.ts

@@ -23,12 +23,12 @@ function getPluginsBaseDir(): string {
   return join(homedir(), ".claude", "plugins")
 }
 
-function getInstalledPluginsPath(): string {
-  return join(getPluginsBaseDir(), "installed_plugins.json")
+function getInstalledPluginsPath(pluginsBaseDir?: string): string {
+  return join(pluginsBaseDir ?? getPluginsBaseDir(), "installed_plugins.json")
 }
 
-function loadInstalledPlugins(): InstalledPluginsDatabase | null {
-  const dbPath = getInstalledPluginsPath()
+function loadInstalledPlugins(pluginsBaseDir?: string): InstalledPluginsDatabase | null {
+  const dbPath = getInstalledPluginsPath(pluginsBaseDir)
   if (!existsSync(dbPath)) {
     return null
   }
@@ -163,7 +163,9 @@ function extractPluginEntries(
 }
 
 export function discoverInstalledPlugins(options?: PluginLoaderOptions): PluginLoadResult {
-  const db = loadInstalledPlugins()
+  // Allow overriding the plugins base directory for testing
+  const pluginsBaseDir = options?.pluginsHomeOverride ?? getPluginsBaseDir()
+  const db = loadInstalledPlugins(pluginsBaseDir)
   const settings = loadClaudeSettings()
   const plugins: LoadedPlugin[] = []
   const errors: PluginLoadError[] = []

src/features/claude-code-plugin-loader/types.ts

@@ -221,6 +221,12 @@ export interface ClaudeSettings {
  * Plugin loader options
  */
 export interface PluginLoaderOptions {
+  /**
+   * Override the plugins home directory for testing.
+   * If not provided, uses CLAUDE_PLUGINS_HOME env var or ~/.claude/plugins
+   */
+  pluginsHomeOverride?: string
+
   /**
    * Override enabled plugins from oh-my-opencode config.
    * Key format: "pluginName@marketplace" (e.g., "shell-scripting@claude-code-workflows")