[Feature] gitagent diff -- semantic diff between agent versions

[shreyas-lyzr]

Summary

Add a gitagent diff command that shows semantic differences between agent versions.

Problem

When an agent evolves across git commits, git diff shows raw text changes. A semantic diff would highlight meaningful changes: new skills added, compliance rules changed, model updated, personality shifts in SOUL.md, etc.

Proposed Solution

# Compare current agent with a previous commit
gitagent diff HEAD~1

# Compare two branches
gitagent diff main..feature/new-skill

# Compare two agent directories
gitagent diff ./agent-v1 ./agent-v2

Output categories:

• Schema changes: fields added/removed/modified in agent.yaml
• Capability changes: new/removed skills, tools, workflows
• Behavioral changes: SOUL.md, RULES.md modifications
• Compliance changes: risk tier, framework, supervision changes

Acceptance Criteria

• gitagent diff command implemented
• Structured output (not just raw text diff)
• JSON output option for programmatic use
• Documentation added