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/constants.ts

@@ -261,12 +261,16 @@ You are NOT an interactive assistant. You are an autonomous problem-solver.
 4. DO NOT ask clarifying questions - the goal is already defined
 
 **Autonomous executor mindset**:
-- You receive a GOAL, not step-by-step instructions
+- You receive a GOAL. When the goal includes numbered steps or phases, treat them as one atomic task broken into sub-steps - NOT as separate independent tasks.
 - Figure out HOW to achieve the goal yourself
 - Thorough research before any action
 - Fix hairy problems that require deep understanding
 - Work independently without frequent check-ins
 
+**Single vs. multi-step context**:
+- Sub-steps of ONE goal (e.g., "Step 1: analyze X, Step 2: implement Y, Step 3: test Z" for a single feature) = execute all steps, they are phases of one atomic task.
+- Genuinely independent tasks (e.g., "Task A: refactor module X" AND "Task B: fix unrelated bug Y") = flag and refuse, require separate delegations.
+
 **Approach**:
 - Explore extensively, understand deeply, then act decisively
 - Prefer comprehensive solutions over quick patches

src/tools/delegate-task/subagent-resolver.ts

@@ -17,7 +17,8 @@ export async function resolveSubagentExecution(
   args: DelegateTaskArgs,
   executorCtx: ExecutorContext,
   parentAgent: string | undefined,
-  categoryExamples: string
+  categoryExamples: string,
+  inheritedModel?: string
 ): Promise<{ agentToUse: string; categoryModel: { providerID: string; modelID: string; variant?: string } | undefined; fallbackChain?: FallbackEntry[]; error?: string }> {
   const { client, agentOverrides, userCategories } = executorCtx
 
@@ -116,7 +117,7 @@ Create the work plan directly - that's your job as the planning agent.`,
         : undefined
 
       const resolution = resolveModelForDelegateTask({
-        userModel: agentOverride?.model,
+        userModel: agentOverride?.model ?? inheritedModel,
         userFallbackModels: normalizedAgentFallbackModels,
         categoryDefaultModel: matchedAgentModelStr,
         fallbackChain: agentRequirement?.fallbackChain,

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)],

src/tools/delegate-task/tools.ts

@@ -226,7 +226,7 @@ export function createDelegateTask(options: DelegateTaskToolOptions): ToolDefini
           return executeUnstableAgentTask(args, ctx, options, parentContext, agentToUse, categoryModel, systemContent, actualModel)
         }
       } else {
-        const resolution = await resolveSubagentExecution(args, options, parentContext.agent, categoryExamples)
+        const resolution = await resolveSubagentExecution(args, options, parentContext.agent, categoryExamples, inheritedModel)
         if (resolution.error) {
           return resolution.error
         }