fix(cli): preserve user config on reinstall

Previously, the install command would delete the entire 'agents' object
from the user's oh-my-opencode config before merging new install settings.
This caused all user customizations to be lost on reinstall.

Fixed by removing the 'delete existing.agents' line and relying on the
existing deepMerge function to properly merge configs, preserving user
customizations while updating only the fields specified by the installer.

Fixes #556

src/cli/config-manager.ts

@@ -341,7 +341,6 @@ export function writeOmoConfig(installConfig: InstallConfig): ConfigMergeResult
           return { success: true, configPath: omoConfigPath }
         }
 
-        delete existing.agents
         const merged = deepMerge(existing, newConfig)
         writeFileSync(omoConfigPath, JSON.stringify(merged, null, 2) + "\n")
       } catch (parseErr) {