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/migrate-legacy-plugin-entry.ts

@@ -0,0 +1,27 @@
+import { existsSync, readFileSync, writeFileSync } from "node:fs"
+
+import { log } from "./logger"
+import { LEGACY_PLUGIN_NAME, PLUGIN_NAME } from "./plugin-identity"
+
+export function migrateLegacyPluginEntry(configPath: string): boolean {
+  if (!existsSync(configPath)) return false
+
+  try {
+    const content = readFileSync(configPath, "utf-8")
+    if (!content.includes(LEGACY_PLUGIN_NAME)) return false
+
+    const updated = content.replaceAll(LEGACY_PLUGIN_NAME, PLUGIN_NAME)
+    if (updated === content) return false
+
+    writeFileSync(configPath, updated, "utf-8")
+    log("[migrateLegacyPluginEntry] Auto-migrated opencode.json plugin entry", {
+      configPath,
+      from: LEGACY_PLUGIN_NAME,
+      to: PLUGIN_NAME,
+    })
+    return true
+  } catch (error) {
+    log("[migrateLegacyPluginEntry] Failed to migrate opencode.json", { configPath, error })
+    return false
+  }
+}