Umbrella behaviour for pluggable presentation shells.

A shell owns layout, chrome, input routing, buffer lifecycle, and tab queries. Each responsibility is a focused sub-behaviour:

• MingaEditor.Shell.Layout — compute_layout/1
• MingaEditor.Shell.Chrome — build_chrome/4, render/1
• MingaEditor.Shell.InputRouter — input_handlers/1, handle_event/3, handle_gui_action/3
• MingaEditor.Shell.BufferLifecycle — on_buffer_added/5, on_buffer_switched/2, on_buffer_died/3, on_agent_event/4
• MingaEditor.Shell.TabQueries — active_tab/1, find_tab_by_buffer/2, active_tab_kind/1, set_tab_session/3, active_session/1

This umbrella declares only init/1 (the constructor every shell needs); all other callbacks live on their respective sub-behaviours. Existing shells (Traditional, Board) implement the umbrella plus every sub-behaviour. A future tab-less shell can implement just the sub-behaviours it cares about (e.g., skip TabQueries entirely if no tab UI is rendered) and EditorState/AgentAccess callers that go through Shell.active_session/1 will continue to work because they read through the field, not the type.

Implementation

Implement init/1 plus the sub-behaviours your shell needs, then set the module as the :shell field on MingaEditor.State. The Editor GenServer dispatches to the active shell for presentation concerns.

Available shells

• MingaEditor.Shell.Traditional — tab-based editor with file tree, modeline, picker, and agent panel. The default shell.
• MingaEditor.Shell.Board — agent supervisor card view.