Add content validate command for Page Designer metadefinitions

[clavery]

Summary

• Adds b2c content validate CLI command that validates Page Designer metadefinition JSON files against bundled JSON Schema draft-04 schemas
• Bundles 19 schemas from B2C documentation covering all metadefinition types (pagetype, componenttype, aspecttype, cmsrecord, etc.)
• Auto-detects schema type from file path conventions and JSON content heuristics, with explicit --type override
• Supports glob patterns, directory expansion, --json output, and colored PASS/FAIL results
• No instance connection or authentication required — purely local validation

Test plan

• SDK tests pass: 32 new tests covering type detection, validation, $ref resolution, error cases
• CLI tests pass: 6 new tests covering output formatting, JSON mode, flag passthrough, glob expansion
• Manual test: ./cli content validate <path-to-metadefinition> with valid and invalid files
• CI passes (lint, typecheck, full test suite)