fix(#2823): auto-migrate legacy plugin name and warn users at startup

- logLegacyPluginStartupWarning now emits console.warn (visible to user,
  not just log file) when oh-my-opencode is detected in opencode.json
- Auto-migrates opencode.json plugin entry from oh-my-opencode to
  oh-my-openagent (with backup)
- plugin-config.ts: add console.warn when loading legacy config filename
- test: 10 tests covering migration, console output, edge cases

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

@@ -1,5 +1,6 @@
 import { checkForLegacyPluginEntry } from "./legacy-plugin-warning"
 import { log } from "./logger"
+import { migrateLegacyPluginEntry } from "./migrate-legacy-plugin-entry"
 import { LEGACY_PLUGIN_NAME, PLUGIN_NAME } from "./plugin-identity"
 
 function toCanonicalEntry(entry: string): string {
@@ -20,9 +21,27 @@ export function logLegacyPluginStartupWarning(): void {
     return
   }
 
+  const suggestedEntries = result.legacyEntries.map(toCanonicalEntry)
+
   log("[OhMyOpenCodePlugin] Legacy plugin entry detected in OpenCode config", {
     legacyEntries: result.legacyEntries,
-    suggestedEntries: result.legacyEntries.map(toCanonicalEntry),
+    suggestedEntries,
     hasCanonicalEntry: result.hasCanonicalEntry,
   })
+
+  console.warn(
+    `[oh-my-openagent] WARNING: Your opencode.json uses the legacy package name "${LEGACY_PLUGIN_NAME}".`
+    + ` The package has been renamed to "${PLUGIN_NAME}".`
+    + ` Attempting auto-migration...`,
+  )
+
+  const migrated = migrateLegacyPluginEntry(result.configPath!)
+  if (migrated) {
+    console.warn(`[oh-my-openagent] Auto-migrated opencode.json: ${result.legacyEntries.join(", ")} -> ${suggestedEntries.join(", ")}`)
+  } else {
+    console.warn(
+      `[oh-my-openagent] Could not auto-migrate. Please manually update your opencode.json:`
+      + ` ${result.legacyEntries.map((e, i) => `"${e}" -> "${suggestedEntries[i]}"`).join(", ")}`,
+    )
+  }
 }