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/010_card_messages.sql

@@ -0,0 +1,14 @@
+-- Per-card chat messages (human-to-human comments).
+-- Distinct from card_events (which records system events like title_changed)
+-- and from /api/chat (which is the board-level LLM chat).
+
+CREATE TABLE IF NOT EXISTS card_messages (
+    id          TEXT PRIMARY KEY,
+    card_id     TEXT NOT NULL,
+    author_id   TEXT,
+    body        TEXT NOT NULL,
+    created_at  TEXT NOT NULL,
+    FOREIGN KEY (card_id) REFERENCES cards(id) ON DELETE CASCADE
+);
+
+CREATE INDEX IF NOT EXISTS idx_card_messages_card ON card_messages(card_id, created_at);