🤖 refactor: remove plan subagent auto-handoff

[ammar-agent]

Summary

Remove plan-mode subagent auto-handoff after propose_plan, along with the executor-routing settings and backend router that supported it.

Background

Plan auto-handoff originally let plan-mode subagents compact into a plan and continue under exec or orchestrator. In practice it now collides with grouped task workflows: planning-only best-of batches can silently turn into implementation work. Removing the feature restores a clearer boundary: planning stays top-level, and task subagents stay implementation-oriented.

Implementation

• removed the taskService stream-end handoff path that compacted history, switched agents, and posted synthetic Implement the plan / orchestrator kickoff messages
• removed the plan-to-executor routing config surface from task settings, the settings UI, and the dedicated router/status modules
• made the built-in plan agent non-runnable as a subagent again and added a task creation guard that rejects any plan-like subagent, including custom agents that inherit from plan
• deleted the obsolete plan auto-routing story and updated orchestrator/docs copy to direct complex planning back to top-level plan workspaces instead of subagent auto-handoff

Validation

• bun test src/common/types/tasks.test.ts
• bun test src/node/services/taskService.test.ts
• make static-check

Risks

Low-to-moderate. This intentionally removes an existing subagent workflow, so users relying on plan subagents that automatically continue into implementation will need to do planning in a top-level workspace instead. The change reduces surprising execution fan-out, and the new creation-time rejection keeps custom plan-like subagents from falling into an unsupported half-working state.

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Generated with mux • Model: openai:gpt-5.4 • Thinking: high • Cost: $2.78

[ammar-agent]

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mux/src/node/services/taskService.ts

Lines 3076 to 3080 in bb69d48

	if (status !== "awaiting_report") {
	await this.setTaskStatus(workspaceId, "awaiting_report");
	}
	await this.promptTaskForRequiredCompletionTool(workspaceId, { reason: "stream_end" });

Treat successful propose_plan as terminal task completion

This path now always pushes plan-like tasks into awaiting_report recovery without first checking whether event.parts already contains a successful propose_plan result, so legacy/in-flight plan-like subagent tasks (e.g. after upgrading with existing tasks) can get stuck in a loop where they are repeatedly told to call propose_plan and never finalize or clean up. Before this change, a successful propose_plan stream-end had a dedicated terminal path; removing it without a replacement leaves affected tasks non-terminating.

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