Replaces single-section AI-only stub with three sections (Project Context,
Currently Assigned, Pick a Mentor). Pick a Mentor is a tab strip:
- Manual Picker (default): all MENTOR-role users sorted by expertise
overlap %, with search + load/capacity columns. Assign sends
method=MANUAL.
- AI Suggestions: existing pane, with an amber 'AI matching unavailable'
banner + 'Tag overlap' pills when OPENAI_API_KEY is unset.
Plan: docs/superpowers/plans/2026-04-28-pr4-mentor-assignment-ux.md