Add validate Command for Configuration Syntax Checking

[jtdub]

Problem

Users may want to validate configuration file syntax without generating remediation output. Currently, syntax errors are only discovered when running remediation/rollback/future commands.

Proposed Solution

Add a "validate" command for syntax checking:

hier-config-cli validate --platform ios --config running.conf

Exit Codes

• 0 - Configuration is valid
• 1 - Configuration has errors
• 2 - File not found or unreadable

Use Cases

• Pre-commit hooks
• CI/CD pipeline validation
• Bulk validation of config templates
• Debugging config generation scripts

Acceptance Criteria

• validate command implemented
• Clear error messages with line numbers
• Appropriate exit codes
• JSON output format option
• Documentation added
• Tests added