Prompt for TUI Homework:
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/wf-ideate Create a TUI using Textual for the second brain CLI.
The TUI should be the default command — running second_brain
with no arguments launches it.
Two-pane layout: left sidebar with a scrollable note list
and a Create button at the bottom, right side is a markdown editor.
Selecting a note shows its content (read-only).
Create button switches to edit mode with Save/Cancel buttons.
Plan first.
