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/tools/delegate-task/sync-prompt-sender.ts

@@ -56,7 +56,9 @@ export async function sendSyncPrompt(
   const promptArgs = {
     path: { id: input.sessionID },
     body: {
-      agent: input.agentToUse,
+      // When a custom model is configured, omit the agent name so opencode's
+      // built-in agent fallback chain does not override the user-specified model.
+      ...(input.categoryModel ? {} : { agent: input.agentToUse }),
       system: input.systemContent,
       tools,
       parts: [createInternalAgentTextPart(effectivePrompt)],