fix(hooks): reuse shared legacy plugin migration helper

Replace the legacy toast regex writer with the shared atomic helper and route legacy plugin call sites through small wrappers so existing mock.module tests stop leaking across the suite.

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

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

src/shared/log-legacy-plugin-startup-warning.test.ts

@@ -26,7 +26,7 @@ mock.module("./logger", () => ({
   log: mockLog,
 }))
 
-mock.module("./migrate-legacy-plugin-entry", () => ({
+mock.module("./plugin-entry-migrator", () => ({
   migrateLegacyPluginEntry: mockMigrateLegacyPluginEntry,
 }))
 

src/shared/log-legacy-plugin-startup-warning.ts

@@ -1,6 +1,6 @@
 import { checkForLegacyPluginEntry } from "./legacy-plugin-warning"
 import { log } from "./logger"
-import { migrateLegacyPluginEntry } from "./migrate-legacy-plugin-entry"
+import { migrateLegacyPluginEntry } from "./plugin-entry-migrator"
 import { LEGACY_PLUGIN_NAME, PLUGIN_NAME } from "./plugin-identity"
 
 function toCanonicalEntry(entry: string): string {

src/shared/plugin-entry-migrator.ts

@@ -0,0 +1 @@
+export { migrateLegacyPluginEntry } from "./migrate-legacy-plugin-entry"