feat: variable registry implementation (shared config for all solution repos)

[kristopherjturner]

Overview

Implement the centralized variable registry — a YAML-based configuration store that serves as the single source of truth for shared infrastructure parameters consumed by all Azure Local solution repos (AVD, SOFS, loadtools, vmconvert, etc.).

Scope

• Define the canonical variable schema in config/variables.example.yml
• JSON Schema validation for all registry entries
• PowerShell module (src/VariableRegistry.psm1) for reading registry values in solution repos
• Python module (src/variable_registry.py) for reading registry values in Python-based repos
• Integration guide documentation for consuming repos
• Version tagging for breaking changes to registry schema

Acceptance Criteria

• All shared variables (cluster endpoints, subscription IDs, naming prefixes, tag policies) defined in registry
• JSON Schema validation runs on registry load and fails fast on invalid entries
• Both PowerShell and Python consumers implemented and tested
• At least 2 solution repos (AVD + loadtools) integrated with registry in their CI
• Migration guide for repos currently using local variable files

Dependencies

• Parent Epic: [Epic] Azure Local Toolkit Platform Automation Full Build-Out #9
• Existing registry issue: Create master variables registry to standardize variables across all repos #4