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/003_columns_extras.sql

@@ -0,0 +1,6 @@
+-- Columnas extra de `columns` (location, width, wip_limit, is_done).
+-- Antes vivian en ensureColumns Go. Reextraidas a migration por consistencia.
+ALTER TABLE columns ADD COLUMN location  TEXT NOT NULL DEFAULT 'board';
+ALTER TABLE columns ADD COLUMN width     INTEGER NOT NULL DEFAULT 300;
+ALTER TABLE columns ADD COLUMN wip_limit INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE columns ADD COLUMN is_done   INTEGER NOT NULL DEFAULT 0;