feat(email): instructions + contact emails + optional note in mentorship templates

Export getMentorBulkAssignmentTemplate and getTeamMentorIntroductionTemplate,
adding an always-on instructions block, optional team-member/teammate contact
lists, and an optional custom note to both. Covers TDD with 4 new unit tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/mentor-welcome-email.test.ts

@@ -0,0 +1,66 @@
+import { describe, it, expect } from 'vitest'
+import {
+  getMentorBulkAssignmentTemplate,
+  getTeamMentorIntroductionTemplate,
+} from '@/lib/email'
+
+describe('getMentorBulkAssignmentTemplate', () => {
+  it('includes team-member emails, the instructions block, and a custom note', () => {
+    const t = getMentorBulkAssignmentTemplate(
+      'Alice',
+      [
+        {
+          title: 'Reef Project',
+          url: 'https://x/mentor/workspace/1',
+          teamMembers: [{ name: 'Bob', email: 'bob@team.com' }],
+        },
+      ],
+      'https://x/mentor',
+      'Please reach out this week.',
+    )
+    expect(t.html).toContain('bob@team.com')
+    expect(t.html).toContain('How to mentor on MOPC')
+    expect(t.html).toContain('Please reach out this week.')
+    expect(t.text).toContain('bob@team.com')
+    expect(t.text).toContain('How to mentor on MOPC')
+  })
+
+  it('renders without team members or note (backward compatible)', () => {
+    const t = getMentorBulkAssignmentTemplate(
+      'Alice',
+      [{ title: 'P', url: 'https://x/p' }],
+      'https://x/mentor',
+    )
+    expect(t.html).toContain('How to mentor on MOPC')
+    // Custom-note box uses the #fff7ed background; absent when no note passed.
+    expect(t.html).not.toContain('#fff7ed')
+  })
+})
+
+describe('getTeamMentorIntroductionTemplate', () => {
+  it('includes mentor + teammate emails, the instructions block, and a custom note', () => {
+    const t = getTeamMentorIntroductionTemplate(
+      'Bob',
+      'Reef Project',
+      [{ name: 'Alice', email: 'alice@mentor.com' }],
+      'https://x/applicant/mentor',
+      [{ name: 'Carol', email: 'carol@team.com' }],
+      'Welcome aboard!',
+    )
+    expect(t.html).toContain('alice@mentor.com')
+    expect(t.html).toContain('carol@team.com')
+    expect(t.html).toContain('Working with your mentor')
+    expect(t.html).toContain('Welcome aboard!')
+  })
+
+  it('renders without teammates or note (backward compatible)', () => {
+    const t = getTeamMentorIntroductionTemplate(
+      'Bob',
+      'Reef Project',
+      [{ name: 'Alice', email: 'alice@mentor.com' }],
+      'https://x/applicant/mentor',
+    )
+    expect(t.html).toContain('Working with your mentor')
+    expect(t.html).not.toContain('#fff7ed')
+  })
+})