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/007_card_events.sql

@@ -0,0 +1,11 @@
+-- Eventos cronologicos por card. Complementa column_history (moves) y lock_history (locks).
+-- Captura: created, assigned, unassigned, title_changed, description_changed, color_changed, tags_changed.
+CREATE TABLE IF NOT EXISTS card_events (
+    id         TEXT PRIMARY KEY,
+    card_id    TEXT NOT NULL REFERENCES cards(id) ON DELETE CASCADE,
+    kind       TEXT NOT NULL,
+    actor_id   TEXT,
+    payload    TEXT NOT NULL DEFAULT '{}',
+    created_at TEXT NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idx_card_events_card ON card_events(card_id, created_at);