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/hooks/legacy-plugin-toast/auto-migrate.ts

@@ -0,0 +1,81 @@
+import { existsSync, readFileSync, writeFileSync } from "node:fs"
+
+import { parseJsoncSafe } from "../../shared/jsonc-parser"
+import { getOpenCodeConfigPaths } from "../../shared/opencode-config-dir"
+import { LEGACY_PLUGIN_NAME, PLUGIN_NAME } from "../../shared/plugin-identity"
+
+export interface MigrationResult {
+  migrated: boolean
+  from: string | null
+  to: string | null
+  configPath: string | null
+}
+
+interface OpenCodeConfig {
+  plugin?: string[]
+}
+
+function isLegacyEntry(entry: string): boolean {
+  return entry === LEGACY_PLUGIN_NAME || entry.startsWith(`${LEGACY_PLUGIN_NAME}@`)
+}
+
+function isCanonicalEntry(entry: string): boolean {
+  return entry === PLUGIN_NAME || entry.startsWith(`${PLUGIN_NAME}@`)
+}
+
+function toLegacyCanonical(entry: string): string {
+  if (entry === LEGACY_PLUGIN_NAME) return PLUGIN_NAME
+  if (entry.startsWith(`${LEGACY_PLUGIN_NAME}@`)) {
+    return `${PLUGIN_NAME}${entry.slice(LEGACY_PLUGIN_NAME.length)}`
+  }
+  return entry
+}
+
+function detectOpenCodeConfigPath(): string | null {
+  const paths = getOpenCodeConfigPaths({ binary: "opencode", version: null })
+  if (existsSync(paths.configJsonc)) return paths.configJsonc
+  if (existsSync(paths.configJson)) return paths.configJson
+  return null
+}
+
+export function autoMigrateLegacyPluginEntry(): MigrationResult {
+  const configPath = detectOpenCodeConfigPath()
+  if (!configPath) return { migrated: false, from: null, to: null, configPath: null }
+
+  try {
+    const content = readFileSync(configPath, "utf-8")
+    const parseResult = parseJsoncSafe<OpenCodeConfig>(content)
+    if (!parseResult.data?.plugin) return { migrated: false, from: null, to: null, configPath }
+
+    const plugins = parseResult.data.plugin
+    const legacyEntries = plugins.filter(isLegacyEntry)
+    if (legacyEntries.length === 0) return { migrated: false, from: null, to: null, configPath }
+
+    const hasCanonical = plugins.some(isCanonicalEntry)
+    const from = legacyEntries[0]
+    const to = toLegacyCanonical(from)
+
+    const normalized = hasCanonical
+      ? plugins.filter((p) => !isLegacyEntry(p))
+      : plugins.map((p) => (isLegacyEntry(p) ? toLegacyCanonical(p) : p))
+
+    const isJsonc = configPath.endsWith(".jsonc")
+    if (isJsonc) {
+      const pluginArrayRegex = /((?:"plugin"|plugin)\s*:\s*)\[([\s\S]*?)\]/
+      const match = content.match(pluginArrayRegex)
+      if (match) {
+        const formattedPlugins = normalized.map((p) => `"${p}"`).join(",\n    ")
+        const newContent = content.replace(pluginArrayRegex, `$1[\n    ${formattedPlugins}\n  ]`)
+        writeFileSync(configPath, newContent)
+        return { migrated: true, from, to, configPath }
+      }
+    }
+
+    const parsed = JSON.parse(content) as Record<string, unknown>
+    parsed.plugin = normalized
+    writeFileSync(configPath, JSON.stringify(parsed, null, 2) + "\n")
+    return { migrated: true, from, to, configPath }
+  } catch {
+    return { migrated: false, from: null, to: null, configPath }
+  }
+}