test(native-chat): pin the strip's monitor amber to the constant that defines it

`MONITOR_GLYPH_COLOR`'s comment claimed a test held it and AgentStateDot's amber
together, but no test imported it — the assertions hardcoded 'text-yellow-500',
so the two could drift with every test still green. Read the colour from the
module, which is what the comment always said was happening. Drop the unused
`BackgroundTaskGlyph` export too: nothing outside the module names it.

src/renderer/src/components/native-chat/NativeChatBackgroundTasksStatus.test.tsx

@@ -6,6 +6,7 @@ import { cleanup, fireEvent, render, screen } from '@testing-library/react'
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import type { AgentSessionBackgroundTask } from '../../../../shared/agent-session-wire'
 import { NativeChatBackgroundTasksStatus } from './NativeChatBackgroundTasksStatus'
+import { MONITOR_GLYPH_COLOR } from './native-chat-background-task-kinds'
 import { AgentStateDot } from '@/components/AgentStateDot'
 
 afterEach(cleanup)
@@ -86,16 +87,18 @@ describe('NativeChatBackgroundTasksStatus row glyphs', () => {
   })
 
   it('gives the monitor heartbeat the amber the monitoring dot uses', () => {
-    // Same glyph on a muted grey would not read as the same thing.
-    expect(glyphSlotClassFor('monitor')).toContain('text-yellow-500')
+    // Same glyph on a muted grey would not read as the same thing. Reading the
+    // colour from the module is what actually pins the two together: a hardcoded
+    // string here would let the strip and the dot drift apart silently.
+    expect(glyphSlotClassFor('monitor')).toContain(MONITOR_GLYPH_COLOR)
     cleanup()
     const { container } = render(<AgentStateDot state="monitoring" size="md" title={null} />)
-    expect(container.querySelector('svg')?.getAttribute('class')).toContain('text-yellow-500')
+    expect(container.querySelector('svg')?.getAttribute('class')).toContain(MONITOR_GLYPH_COLOR)
   })
 
   it('leaves the other kinds muted, so only monitoring reads as a state', () => {
     for (const kind of ['agent', 'command', 'workflow', 'unknown'] as const) {
-      expect(glyphSlotClassFor(kind)).not.toContain('text-yellow-500')
+      expect(glyphSlotClassFor(kind)).not.toContain(MONITOR_GLYPH_COLOR)
       cleanup()
     }
   })

src/renderer/src/components/native-chat/native-chat-background-task-kinds.tsx

@@ -18,7 +18,7 @@ type BackgroundTaskKind = AgentSessionBackgroundTask['kind']
 /** `heartbeat` is deliberately outside the tool table: monitoring is one idea
  *  app-wide, and the heartbeat is the glyph that already stands for it in the
  *  agent sidebar and on AgentStateDot. A second monitor glyph would split it. */
-export type BackgroundTaskGlyph = NativeChatToolIconName | 'heartbeat'
+type BackgroundTaskGlyph = NativeChatToolIconName | 'heartbeat'
 
 const KIND_GLYPHS: Record<BackgroundTaskKind, BackgroundTaskGlyph> = {
   agent: 'bot',