feat(graph): add Assign button and archetype dropdown to GraphNodeCard

## The Collaboration Story
User requested ability to assign agent archetypes to tasks directly from
graph nodes. This was the core UI feature request.

## Design Decisions Made Together
1. **Placement**: We decided to put the assign UI at the bottom of the card
   to not interfere with existing status/badges display
2. **Pattern**: Used Radix dropdown-menu (already in project) for consistency
3. **Visual feedback**: Added loading spinner during API calls, success/error
   messages that auto-dismiss
4. **Closed tasks**: Excluded closed tasks from assignment (logical constraint)

## Issues We Encountered
- Initially only added POST handler for assignment
- User pointed out we needed DELETE for unassign - added that
- The unassign button (X) needed to call DELETE not POST

## API Changes
- Added DELETE handler to /api/swarm/prep for removing agent assignments
- Uses 'bd label remove' under the hood

## What the UI Shows
- Dropdown with available archetypes
- Badge showing assigned archetype name with color
- X button to unassign (on each badge)
- 'Assigned' label on already-assigned archetypes in dropdown

## Test Coverage
- Added graph-node-assign.test.tsx with 6 TDD tests

## Beads: beadboard-brq (closed)

tests/components/graph/graph-node-assign.test.tsx

@@ -0,0 +1,66 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import fs from 'fs/promises';
+import path from 'path';
+
+// Test that GraphNodeCard has Assign button for assignable tasks
+test('GraphNodeCard checks for assignable status (open, blocked, ready)', async () => {
+  const fileContent = await fs.readFile(path.join(process.cwd(), 'src/components/graph/graph-node-card.tsx'), 'utf-8');
+  assert.ok(
+    fileContent.includes("'open'") && 
+    (fileContent.includes("'blocked'") || fileContent.includes("status === 'blocked'")),
+    'GraphNodeCard should check for open/blocked status for assign button'
+  );
+});
+
+// Test that Assign button is NOT shown for closed tasks
+test('GraphNodeCard excludes closed tasks from assign button', async () => {
+  const fileContent = await fs.readFile(path.join(process.cwd(), 'src/components/graph/graph-node-card.tsx'), 'utf-8');
+  assert.ok(
+    !fileContent.includes("status === 'closed' &&") || 
+    fileContent.includes("status !== 'closed'"),
+    'Assign button should not show for closed tasks'
+  );
+});
+
+// Test that GraphNodeCard shows assigned archetype from labels
+test('GraphNodeCard parses agent: label to show assigned archetype', async () => {
+  const fileContent = await fs.readFile(path.join(process.cwd(), 'src/components/graph/graph-node-card.tsx'), 'utf-8');
+  assert.ok(
+    fileContent.includes('agent:') || 
+    fileContent.includes('getArchetypeFromLabels') ||
+    fileContent.includes('data.labels'),
+    'GraphNodeCard should check for agent: labels'
+  );
+});
+
+// Test that Radix dropdown-menu is imported
+test('GraphNodeCard imports Radix dropdown-menu for archetype selection', async () => {
+  const fileContent = await fs.readFile(path.join(process.cwd(), 'src/components/graph/graph-node-card.tsx'), 'utf-8');
+  assert.ok(
+    fileContent.includes('@radix-ui/react-dropdown-menu') ||
+    fileContent.includes('DropdownMenu'),
+    'GraphNodeCard should import Radix dropdown-menu'
+  );
+});
+
+// Test that archetypes are passed to node and used
+test('GraphNodeCard receives and uses archetypes for dropdown', async () => {
+  const fileContent = await fs.readFile(path.join(process.cwd(), 'src/components/graph/graph-node-card.tsx'), 'utf-8');
+  assert.ok(
+    fileContent.includes('archetypes') || 
+    fileContent.includes('AgentArchetype'),
+    'GraphNodeCard should reference archetypes'
+  );
+});
+
+// Test that onAssign callback or similar is supported
+test('GraphNodeCard supports assignment callback', async () => {
+  const fileContent = await fs.readFile(path.join(process.cwd(), 'src/components/graph/graph-node-card.tsx'), 'utf-8');
+  assert.ok(
+    fileContent.includes('onAssign') || 
+    fileContent.includes('Assign') ||
+    fileContent.includes('/api/swarm/prep'),
+    'GraphNodeCard should support assignment action'
+  );
+});