feat: implement graph diff, 3-way merge, and auto-merge (#348-PR2-8)

Summary

Implement the complete #348 roadmap for node-level graph merging:

• PR2: Node-level "what changed" diff

  • zero graph diff --base <a> <b> --json
  • Returns changeset with added/removed/modified entries

• PR3: 3-way collision detection

  • zero graph merge --base <base> <ours> <theirs> --json
  • Reports conflicts with GMG### codes

• PR4: Auto-combine non-overlapping edits

  • Same command with --out <file> to write merged result
  • Re-validates merged graph

• PR5: Trivial convergences

  • Auto-resolve identical edits, delete/delete, add/add
  • Order-insensitive list handling

• PR6: Field-level disjointness

  • Same node, different fields = no conflict
  • Only same field = GMG001 conflict

• PR7: Move-aware resolution

  • Track node moves via symbol_id changes
  • Handle parent-moved/child-edited cases
  • Detect move/move cycles (GMG005, GMG006)

• PR8: Semantic post-merge checks

  • Run validation and type-checking on merged graph
  • Report issues without auto-fixing

Acceptance Criteria

• Diff: value edit shows as single modified entry
• Merge: disjoint edits → successful merge
• Merge: conflicts → reported with GMG codes
• Auto-resolve trivial convergences
• Field-level disjointness handled
• Move-aware resolution implemented
• Semantic validation on merged result