feat: initial scaffold kanban_cpp v0.1.0

C++ ImGui kanban for steering LLM agents. Six panels (Board, Calendar,
Dashboard, Agent runs, Worktrees, DoD inspector) wired to registry
functions http_request, kpi_card, sparkline, agent_runs_timeline,
dod_evidence_panel. Backend Go on :8403 (independent operations.db from
kanban_web).

backend/migrations/005_history_actor.sql

@@ -0,0 +1,3 @@
+-- actor_id en histories (quien movió la card / quien bloqueó).
+ALTER TABLE card_column_history ADD COLUMN actor_id TEXT;
+ALTER TABLE card_lock_history   ADD COLUMN actor_id TEXT;