## Context
This commit adds the supporting infrastructure that makes the assign
feature work end-to-end.
## Components Added/Modified
### SmartDag
- Main view component for graph-based task management
- Integrates TaskCardGrid and WorkflowGraph
- Has 'Assign' mode toggle button
- Passes archetypes and assignMode to WorkflowGraph
- Manages filter state (hideClosed, sortReadyFirst, etc.)
### useGraphAnalysis Hook
- Extracted graph analysis logic for reuse
- Returns: actionableNodeIds, cycleNodeIdSet, blockerTooltipMap, etc.
- Used by both SmartDag and AssignmentPanel
- Ensures consistent 'actionable' definition across components
### UnifiedShell
- Added assignMode state
- Added selectedAssignIssue state
- Renders AssignmentPanel when in graph view + assign mode
- Wires up onAssignModeChange and onSelectedIssueChange callbacks
## Design Philosophy
- Shared hook means single source of truth for 'actionable'
- Clean separation between view (SmartDag) and sidebar (AssignmentPanel)
- URL state preserved for navigation
## Test Coverage
- SmartDag tests: 12 tests covering buttons, callbacks, imports
- useGraphAnalysis tests: 6 tests covering cycle detection, blockers
- UnifiedShell tests: 9 tests covering state and rendering
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