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).

panels.h

@@ -0,0 +1,31 @@
+// panels.h — Panel draw functions for kanban_cpp.
+//
+// Each draw_* expects to be called inside an active ImGui frame; it issues
+// its own ImGui::Begin/End block guarded by the supplied bool*.
+#pragma once
+
+#include "data.h"
+
+namespace kanban_cpp {
+
+// Shared app state passed to every panel. Owned by main.cpp.
+struct AppState {
+    ClientConfig cfg;
+    std::vector<Card>    cards;
+    std::vector<Column>  columns;
+    std::string last_refresh_error;
+    int64_t     last_refresh_ts = 0;
+    bool        backend_ok = false;
+};
+
+void draw_board     (AppState& s, bool* p_open);
+void draw_calendar  (AppState& s, bool* p_open);
+void draw_dashboard (AppState& s, bool* p_open);
+void draw_agent_runs(AppState& s, bool* p_open);
+void draw_worktrees (AppState& s, bool* p_open);
+void draw_dod       (AppState& s, bool* p_open);
+
+// Polls the backend for cards/columns; updates s.last_refresh_*.
+void refresh_data(AppState& s);
+
+} // namespace kanban_cpp