refactor(plugin): remove orphaned createPluginDispose + stale test mocks

Remove the dead plugin-dispose module and its dedicated test now that V1 plugin migration removed the last production call site. Clean the remaining bootstrap test mocks so src no longer references createPluginDispose.

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

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

src/plugin-dispose.ts

@@ -1,51 +0,0 @@
-import { log } from "./shared"
-
-export type PluginDispose = () => Promise<void>
-
-export function createPluginDispose(args: {
-  backgroundManager: {
-    shutdown: () => void | Promise<void>
-  }
-  skillMcpManager: {
-    disconnectAll: () => Promise<void>
-  }
-  lspManager: {
-    stopAll: () => Promise<void>
-  }
-  disposeHooks: () => void
-}): PluginDispose {
-  const { backgroundManager, skillMcpManager, lspManager, disposeHooks } = args
-  let disposePromise: Promise<void> | null = null
-
-  return async (): Promise<void> => {
-    if (disposePromise) {
-      await disposePromise
-      return
-    }
-
-    disposePromise = (async (): Promise<void> => {
-      try {
-        await backgroundManager.shutdown()
-      } catch (error) {
-        log("[plugin-dispose] backgroundManager.shutdown() error:", error)
-      }
-      try {
-        await skillMcpManager.disconnectAll()
-      } catch (error) {
-        log("[plugin-dispose] skillMcpManager.disconnectAll() error:", error)
-      }
-      try {
-        await lspManager.stopAll()
-      } catch (error) {
-        log("[plugin-dispose] lspManager.stopAll() error:", error)
-      }
-      try {
-        disposeHooks()
-      } catch (error) {
-        log("[plugin-dispose] disposeHooks() error:", error)
-      }
-    })()
-
-    await disposePromise
-  }
-}