plan: add tier2 incremental kuzu rebuild plan (#263)

Four-PR plan (T1-T4) derived from propose/active/TIER2-INCREMENTAL-REBUILD-PROPOSE.md:
dependency index, symmetric delete helpers, incremental orchestrator,
CLI + decision engine integration. Reviewed for codebase accuracy.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: HumanBean17