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/004_cards_extras.sql

@@ -0,0 +1,9 @@
+-- Columnas extra de `cards` (color, locked, assignee_id, completed_at, deleted_at, tags).
+-- Antes vivian en ensureColumns Go. La columna stickers va aparte en 002.
+ALTER TABLE cards ADD COLUMN color        TEXT    NOT NULL DEFAULT '';
+ALTER TABLE cards ADD COLUMN locked       INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE cards ADD COLUMN assignee_id  TEXT;
+ALTER TABLE cards ADD COLUMN completed_at TEXT;
+ALTER TABLE cards ADD COLUMN deleted_at   TEXT;
+ALTER TABLE cards ADD COLUMN tags         TEXT    NOT NULL DEFAULT '[]';
+CREATE INDEX IF NOT EXISTS idx_cards_assignee ON cards(assignee_id);