docs: add data access matrix for TeachLink Mobile governance

[Essence7-01]

Overview

This PR adds the missing Governance/matrices/DATA_ACCESS_MATRIX.md document, closing a governance gap by providing a clear, versioned reference for data classifications, role-based access, and review cadence within the TeachLink Mobile governance folder.

Related Issue

Changes

📋 Governance Documentation

• [ADD] Governance/matrices/DATA_ACCESS_MATRIX.md
  • Documents data classifications and the roles that interact with them.
  • Specifies the allowed access per role.
  • Defines the review cadence for keeping the matrix current.

Verification Results

Documentation-only change:
✅ New file created at Governance/matrices/DATA_ACCESS_MATRIX.md
✅ Scope limited to a single file within the Governance folder
✅ No changes outside Governance/
✅ No code paths modified — no regression risk

Acceptance Criteria	Status
Governance requirement is implemented successfully	✅ Data access matrix document added
Scope is limited to a maximum of two files	✅ One file changed
No changes are made outside the Governance folder	✅ Only Governance/matrices/DATA_ACCESS_MATRIX.md modified
No regression in existing functionality	✅ Documentation-only change
Tests pass and code follows project standards	✅ No code or tests affected
Change is documented	✅ Matrix documents classifications, roles, access, and review cadence

Closes #1453

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