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/plugin-config.ts

@@ -10,6 +10,8 @@ import {
   detectPluginConfigFile,
   migrateConfigFile,
 } from "./shared";
+import { migrateLegacyConfigFile } from "./shared/migrate-legacy-config-file";
+import { LEGACY_CONFIG_BASENAME } from "./shared/plugin-identity";
 
 const PARTIAL_STRING_ARRAY_KEYS = new Set([
   "disabled_mcps",
@@ -168,6 +170,11 @@ export function loadPluginConfig(
       ? userDetected.path
       : path.join(configDir, "oh-my-opencode.json");
 
+  // Auto-copy legacy config file to canonical name if needed
+  if (userDetected.format !== "none" && path.basename(userDetected.path).startsWith(LEGACY_CONFIG_BASENAME)) {
+    migrateLegacyConfigFile(userDetected.path);
+  }
+
   // Project-level config path - prefer .jsonc over .json
   const projectBasePath = path.join(directory, ".opencode");
   const projectDetected = detectPluginConfigFile(projectBasePath);
@@ -176,6 +183,11 @@ export function loadPluginConfig(
       ? projectDetected.path
       : path.join(projectBasePath, "oh-my-opencode.json");
 
+  // Auto-copy legacy project config file to canonical name if needed
+  if (projectDetected.format !== "none" && path.basename(projectDetected.path).startsWith(LEGACY_CONFIG_BASENAME)) {
+    migrateLegacyConfigFile(projectDetected.path);
+  }
+
   // Load user config first (base)
   let config: OhMyOpenCodeConfig =
     loadConfigFromPath(userConfigPath, ctx) ?? {};