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/008_card_seq_num.sql

@@ -0,0 +1,7 @@
+-- ID secuencial humano por card. Distinto del id hex (PK interna).
+-- Backfill por orden de creacion.
+ALTER TABLE cards ADD COLUMN seq_num INTEGER NOT NULL DEFAULT 0;
+UPDATE cards SET seq_num = (
+    SELECT COUNT(*) FROM cards c2 WHERE c2.created_at <= cards.created_at
+) WHERE seq_num = 0;
+CREATE UNIQUE INDEX IF NOT EXISTS idx_cards_seq_num ON cards(seq_num) WHERE seq_num > 0;