cmux sidebar builder

Use this skill when the task is to create or modify a cmux left-sidebar custom view. These are user/agent-authored .swift files in ~/.config/cmux/sidebars/ rendered by cmux's runtime SwiftUI-style interpreter, not compiled app code.

Orientation

• The original work is PR https://github.com/manaflow-ai/cmux/pull/5254 by Aziz Albahar.
• The feature is opt-in behind the UserDefaults-backed key customSidebars.beta.enabled.
• Custom sidebars live under ~/.config/cmux/sidebars/ and appear in the left sidebar picker when the beta is enabled.
• Start from the cmux app checkout docs: repo/docs/custom-sidebars.md.

Important cmux source entrypoints:

• repo/Packages/CmuxSwiftRender/Sources/CmuxSwiftRender/SwiftViewInterpreter.swift
• repo/Packages/CmuxSwiftRender/Sources/CmuxSwiftRender/ExpressionEvaluator.swift
• repo/Packages/CmuxSwiftRender/Sources/CmuxSwiftRender/RenderNode.swift
• repo/Packages/CmuxSwiftRenderUI/Sources/CmuxSwiftRenderUI/Sidebar/CustomSidebarView.swift
• repo/Sources/ContentView.swift, around customSidebarsDirectory, customSidebarDataContext, and CustomSidebarView(...).