[FEATURE] Surface subagent-generated plans in the Desktop Plan panel

Summary

When the main session delegates planning to a subagent (e.g. Agent(subagent_type: "Plan")), the resulting plan is returned to the parent session as text only and never appears in the Claude Desktop Plan panel. The Plan panel populates exclusively when the main session itself calls ExitPlanMode. Subagents have no way to surface a plan to that panel.

Current behavior

• The Plan subagent type is provisioned with all tools except Agent, ExitPlanMode, Edit, Write, NotebookEdit — so it cannot call ExitPlanMode even if instructed to.
• A subagent's plan is returned to the parent as a normal tool result (text). The parent then acts on it (posts to an issue, starts implementing, etc.).
• The Desktop Plan panel stays on "No plan yet" the entire time, because no ExitPlanMode call ever reaches the desktop UI from the subagent path.

This means any workflow that offloads deep planning to an Opus Plan subagent (a common pattern for token efficiency / model routing) loses all Plan-panel visibility — the user cannot review the proposed plan in the dedicated panel before execution.

Expected behavior

A subagent that produces a plan should be able to surface it to the Desktop Plan panel — for example one of:

1. Propagate ExitPlanMode from a subagent to the parent's Plan panel (grant the Plan subagent type ExitPlanMode, and route its plan-proposal up to the desktop UI), or
2. A return-channel where a subagent's structured plan result is rendered in the parent's Plan panel automatically (the parent "adopts" the subagent's plan into the panel without re-calling the tool), or
3. An explicit parent-side affordance to push a received subagent plan into the Plan panel for user review.

The goal: plans created by subagents are reviewable in the same Desktop Plan panel as main-session plans, instead of being invisible there.

Why this matters

• Model routing: Teams route planning to a stronger model via an Opus Plan subagent for cost/quality reasons. Today that choice forces a tradeoff: better planning model or Plan-panel visibility, never both.
• Desktop parity: Desktop users already lose visibility for TodoWrite/task lists in the Tasks panel (#57019). Subagent plans are a second blind spot in the same surface area.
• Review ergonomics: The Plan panel (Reject / Revise / Accept) is the natural review gate. Subagent-authored plans bypass it entirely, so the user can only review plan text inline in the transcript.

Environment

• Surface: Claude Desktop app (Code mode), Plan panel
• Pattern: Agent(subagent_type: "Plan", model: "opus") returning a plan to the main session

Related

• #57019 — Show TodoWrite task list in Claude.app Tasks panel (same Desktop-visibility gap, Tasks panel)
• #62277 — Show subagents of agents in the Agents view
• #60763 — Subagents have no Agent/Task tool (related subagent-capability gap)