feat: make systemDefaultModel optional for OpenCode fallback (#1136)

- Remove mandatory model requirement from plugin initialization
- Allow OpenCode to use its built-in model fallback when user doesn't specify
- Update model-resolver to handle undefined systemDefaultModel
- Remove throw errors in config-handler, utils, atlas, delegate-task
- Add tests for optional model scenarios

Closes #1129

Co-authored-by: justsisyphus <justsisyphus@users.noreply.github.com>

src/plugin-handlers/config-handler.ts

@@ -105,41 +105,6 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
       log(`Plugin load errors`, { errors: pluginComponents.errors });
     }
 
-    if (!(config.model as string | undefined)?.trim()) {
-      let fallbackModel: string | undefined
-
-      for (const agentConfig of Object.values(pluginConfig.agents ?? {})) {
-        const model = (agentConfig as { model?: string })?.model
-        if (model && typeof model === 'string' && model.trim()) {
-          fallbackModel = model.trim()
-          break
-        }
-      }
-
-      if (!fallbackModel) {
-        for (const categoryConfig of Object.values(pluginConfig.categories ?? {})) {
-          const model = (categoryConfig as { model?: string })?.model
-          if (model && typeof model === 'string' && model.trim()) {
-            fallbackModel = model.trim()
-            break
-          }
-        }
-      }
-
-      if (fallbackModel) {
-        config.model = fallbackModel
-        log(`No default model specified, using fallback from config: ${fallbackModel}`)
-      } else {
-        const paths = getOpenCodeConfigPaths({ binary: "opencode", version: null })
-        throw new Error(
-          'oh-my-opencode requires a default model.\n\n' +
-          `Add this to ${paths.configJsonc}:\n\n` +
-          '  "model": "anthropic/claude-sonnet-4-5"\n\n' +
-          '(Replace with your preferred provider/model)'
-        )
-      }
-    }
-
     // Migrate disabled_agents from old names to new names
     const migratedDisabledAgents = (pluginConfig.disabled_agents ?? []).map(agent => {
       return AGENT_NAME_MAP[agent.toLowerCase()] ?? AGENT_NAME_MAP[agent] ?? agent