feat: graph diff between commits (git mind diff)

[flyingrobots]

Summary

git mind diff <ref-a>..<ref-b> — show what changed in the knowledge graph between two git commits.

Materialize the graph at two epoch ceilings, compute set differences on nodes and edges, report additions/removals/property changes.

Use cases

• PR review: "What did this branch do to the knowledge model?"
• CI gates: "Did this refactor increase orphan count?"
• Observability: "How has the graph evolved over the last 10 commits?"

Foundation for

• CI policies (fail on topology regression)
• Review summaries
• Trend metrics
• Future provenance/timeline features

Technical approach

• Two materialize({ ceiling }) calls via epoch system
• Set diff on node/edge collections
• CLI output + --json structured output
• Formatter for human-readable diff

Acceptance criteria

• git mind diff <ref-a>..<ref-b> produces accurate diff
• --json output for CI consumption
• Handles missing epochs (nearest ancestor fallback)
• Tests with deterministic fixtures
• CHANGELOG + GUIDE updated