fix(installer): add upgrade path safety checks

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/cli/config-manager/add-plugin-to-opencode-config.ts

@@ -1,12 +1,14 @@
 import { readFileSync, writeFileSync } from "node:fs"
 import type { ConfigMergeResult } from "../types"
 import { PLUGIN_NAME, LEGACY_PLUGIN_NAME } from "../../shared"
+import { backupConfigFile } from "./backup-config"
 import { getConfigDir } from "./config-context"
 import { ensureConfigDirectoryExists } from "./ensure-config-directory-exists"
 import { formatErrorWithSuggestion } from "./format-error-with-suggestion"
 import { detectConfigFormat } from "./opencode-config-format"
 import { parseOpenCodeConfigFileWithError, type OpenCodeConfig } from "./parse-opencode-config-file"
 import { getPluginNameWithVersion } from "./plugin-name-with-version"
+import { checkVersionCompatibility, extractVersionFromPluginEntry } from "./version-compatibility"
 
 export async function addPluginToOpenCodeConfig(currentVersion: string): Promise<ConfigMergeResult> {
   try {
@@ -52,6 +54,29 @@ export async function addPluginToOpenCodeConfig(currentVersion: string): Promise
         && !(plugin === LEGACY_PLUGIN_NAME || plugin.startsWith(`${LEGACY_PLUGIN_NAME}@`))
     )
 
+    const existingEntry = canonicalEntries[0] ?? legacyEntries[0]
+    if (existingEntry) {
+      const installedVersion = extractVersionFromPluginEntry(existingEntry)
+      const compatibility = checkVersionCompatibility(installedVersion, currentVersion)
+
+      if (!compatibility.canUpgrade) {
+        return {
+          success: false,
+          configPath: path,
+          error: compatibility.reason ?? "Version compatibility check failed",
+        }
+      }
+
+      const backupResult = backupConfigFile(path)
+      if (!backupResult.success) {
+        return {
+          success: false,
+          configPath: path,
+          error: `Failed to create backup: ${backupResult.error}`,
+        }
+      }
+    }
+
     const normalizedPlugins = [...otherPlugins]
 
     if (canonicalEntries.length > 0) {

src/cli/config-manager/detect-current-config.ts

@@ -4,6 +4,7 @@ import type { DetectedConfig } from "../types"
 import { getOmoConfigPath } from "./config-context"
 import { detectConfigFormat } from "./opencode-config-format"
 import { parseOpenCodeConfigFileWithError } from "./parse-opencode-config-file"
+import { extractVersionFromPluginEntry } from "./version-compatibility"
 
 function detectProvidersFromOmoConfig(): {
   hasOpenAI: boolean
@@ -60,9 +61,14 @@ function isOurPlugin(plugin: string): boolean {
          plugin === LEGACY_PLUGIN_NAME || plugin.startsWith(`${LEGACY_PLUGIN_NAME}@`)
 }
 
+function findOurPluginEntry(plugins: string[]): string | null {
+  return plugins.find(isOurPlugin) ?? null
+}
+
 export function detectCurrentConfig(): DetectedConfig {
   const result: DetectedConfig = {
     isInstalled: false,
+    installedVersion: null,
     hasClaude: true,
     isMax20: true,
     hasOpenAI: true,
@@ -86,7 +92,12 @@ export function detectCurrentConfig(): DetectedConfig {
 
   const openCodeConfig = parseResult.config
   const plugins = openCodeConfig.plugin ?? []
-  result.isInstalled = plugins.some(isOurPlugin)
+  const ourPluginEntry = findOurPluginEntry(plugins)
+  result.isInstalled = !!ourPluginEntry
+
+  if (ourPluginEntry) {
+    result.installedVersion = extractVersionFromPluginEntry(ourPluginEntry)
+  }
 
   if (!result.isInstalled) {
     return result

src/cli/config-manager/write-omo-config.test.ts

@@ -18,6 +18,7 @@ const installConfig: InstallConfig = {
   hasOpencodeZen: false,
   hasZaiCodingPlan: false,
   hasKimiForCoding: false,
+  hasOpencodeGo: false,
 }
 
 function getRecord(value: unknown): Record<string, unknown> {

src/cli/config-manager/write-omo-config.ts

@@ -1,6 +1,7 @@
 import { existsSync, readFileSync, statSync, writeFileSync } from "node:fs"
 import { parseJsonc } from "../../shared"
 import type { ConfigMergeResult, InstallConfig } from "../types"
+import { backupConfigFile } from "./backup-config"
 import { getConfigDir, getOmoConfigPath } from "./config-context"
 import { deepMergeRecord } from "./deep-merge-record"
 import { ensureConfigDirectoryExists } from "./ensure-config-directory-exists"
@@ -28,6 +29,15 @@ export function writeOmoConfig(installConfig: InstallConfig): ConfigMergeResult
     const newConfig = generateOmoConfig(installConfig)
 
     if (existsSync(omoConfigPath)) {
+      const backupResult = backupConfigFile(omoConfigPath)
+      if (!backupResult.success) {
+        return {
+          success: false,
+          configPath: omoConfigPath,
+          error: `Failed to create backup: ${backupResult.error}`,
+        }
+      }
+
       try {
         const stat = statSync(omoConfigPath)
         const content = readFileSync(omoConfigPath, "utf-8")