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-config-file.ts

@@ -0,0 +1,31 @@
+import { existsSync, copyFileSync, renameSync } from "node:fs"
+import { join, dirname, basename } from "node:path"
+
+import { log } from "./logger"
+import { CONFIG_BASENAME, LEGACY_CONFIG_BASENAME } from "./plugin-identity"
+
+function buildCanonicalPath(legacyPath: string): string {
+  const dir = dirname(legacyPath)
+  const ext = basename(legacyPath).includes(".jsonc") ? ".jsonc" : ".json"
+  return join(dir, `${CONFIG_BASENAME}${ext}`)
+}
+
+export function migrateLegacyConfigFile(legacyPath: string): boolean {
+  if (!existsSync(legacyPath)) return false
+  if (!basename(legacyPath).startsWith(LEGACY_CONFIG_BASENAME)) return false
+
+  const canonicalPath = buildCanonicalPath(legacyPath)
+  if (existsSync(canonicalPath)) return false
+
+  try {
+    copyFileSync(legacyPath, canonicalPath)
+    log("[migrateLegacyConfigFile] Copied legacy config to canonical path", {
+      from: legacyPath,
+      to: canonicalPath,
+    })
+    return true
+  } catch (error) {
+    log("[migrateLegacyConfigFile] Failed to copy legacy config file", { legacyPath, error })
+    return false
+  }
+}