Migrate graph storage from KuzuDB to LadybugDB

[HumanBean17]

Summary

• Replace the archived kuzu dependency with its maintained fork ladybug (same core team, MIT licensed, actively developed).
• This PR currently contains the proposal (propose/active/LADYBUG-DB-MIGRATE-PROPOSE.md) and the implementation plan (plans/active/PLAN-LADYBUG-DB-MIGRATE.md).
• The implementation will be a single PR with purely mechanical renames across 43 Python files and 11 markdown docs: import kuzu → import ladybug, KuzuGraph → LadybugGraph, kuzu_queries → ladybug_queries, --kuzu-path → --ladybug-path, code_graph.kuzu → code_graph.lbug.

Key decisions

Decision	Choice	Rationale
Compatibility shim	Clean break	Repo allows breaking changes
kuzu_type rename	graph_type	Engine-agnostic, avoids future rename
Dependency pin	ladybug>=0.17.1,<0.18	Mirrors existing strict minor pin
DB file extension	.lbug	Short, unique, no collisions
Ontology bump	None required	No enrichment semantics change

Test plan

• Full test suite passes after mechanical rename (.venv/bin/python -m pytest tests -v)
• Graph builder smoke produces valid .lbug database from bank-chat fixture
• grep -ri "kuzu" --include="*.py" returns zero hits
• grep -ri "kuzu" --include="*.md" AGENTS.md README.md docs/ tests/README.md returns zero hits
• .venv/bin/ruff check . passes

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