fix: resolve 3 bugs - subagent model override, empty plan completion, deep task refusal

- #2741: Pass inheritedModel as fallback in subagent-resolver when user hasn't
  configured an override, ensuring custom provider models take priority
- #2648: Fix getPlanProgress to treat plans with 0 checkboxes as incomplete
  instead of complete (total > 0 && completed === total)
- #2779: Relax Hephaestus single-task guard to accept multi-step sub-tasks
  from Atlas delegation, only rejecting genuinely independent tasks

Fixes #2741, fixes #2648, fixes #2779

src/features/background-agent/spawner.ts

@@ -135,7 +135,9 @@ export async function startTask(
   promptWithModelSuggestionRetry(client, {
     path: { id: sessionID },
     body: {
-      agent: input.agent,
+      // When a model is explicitly provided, omit the agent name so opencode's
+      // built-in agent fallback chain does not override the user-specified model.
+      ...(launchModel ? {} : { agent: input.agent }),
       ...(launchModel ? { model: launchModel } : {}),
       ...(launchVariant ? { variant: launchVariant } : {}),
       system: input.skillContent,
@@ -220,7 +222,9 @@ export async function resumeTask(
   client.session.promptAsync({
     path: { id: task.sessionID },
     body: {
-      agent: task.agent,
+      // When a model is explicitly provided, omit the agent name so opencode's
+      // built-in agent fallback chain does not override the user-specified model.
+      ...(resumeModel ? {} : { agent: task.agent }),
       ...(resumeModel ? { model: resumeModel } : {}),
       ...(resumeVariant ? { variant: resumeVariant } : {}),
       tools: {