Add TUI kanban board, review workflow, and new task statuses

Replace task statuses (pending/running/succeeded/failed/cancelled) with
a kanban workflow: todo → in_progress → in_review → done/wontdo.

When a non-review agent task completes, the API auto-creates a child
review task and moves the parent to in_review. Only humans can move
tasks from in_review to done/wontdo via the TUI.

New components:
- cmd/tui: bubbletea kanban board with $EDITOR integration
- POST /api/v1/tasks/{id}/complete: agent completion callback
- Operator --api-url flag for completion callbacks
- ProviderQueue sets tasks to in_progress on pickup
- AgentTask reconciler calls /complete on job finish

internal/database/migrations.go

@@ -17,7 +17,7 @@ func (db *DB) migrate() error {
 			body            TEXT NOT NULL DEFAULT '',
 			author          TEXT NOT NULL,
 			extra_tools     TEXT[] NOT NULL DEFAULT '{}',
-			status          TEXT NOT NULL DEFAULT 'pending',
+			status          TEXT NOT NULL DEFAULT 'todo',
 			pool_ref        TEXT NOT NULL DEFAULT '',
 			job_name        TEXT NOT NULL DEFAULT '',
 			result          TEXT NOT NULL DEFAULT '',
@@ -30,6 +30,12 @@ func (db *DB) migrate() error {
 		CREATE INDEX IF NOT EXISTS idx_tasks_status ON tasks(status);
 		CREATE INDEX IF NOT EXISTS idx_tasks_repository ON tasks(repository);
 		CREATE INDEX IF NOT EXISTS idx_tasks_parent ON tasks(parent_task_id);
+
+		-- migrate legacy statuses
+		UPDATE tasks SET status = 'todo' WHERE status IN ('pending', 'failed');
+		UPDATE tasks SET status = 'in_progress' WHERE status = 'running';
+		UPDATE tasks SET status = 'done' WHERE status = 'succeeded';
+		UPDATE tasks SET status = 'wontdo' WHERE status = 'cancelled';
 	`)
 	return err
 }