Add diff Command for Configuration Comparison

[jtdub]

Problem

Users need to mentally piece together what changes will be made from remediation output. A visual diff would make it easier to review changes before applying them.

Proposed Solution

Add a new "diff" command showing unified or side-by-side diff between running and intended configs.

Options

• --format unified|side-by-side|context
• --context N (lines of context)
• --color/--no-color

Use Cases

• Code review of configuration changes
• Change approval workflows
• Documentation of planned changes
• Quick sanity check before remediation

Acceptance Criteria

• diff command implemented
• Unified diff format works
• Side-by-side format works
• Color output supported
• Documentation added
• Tests added