feat(tasks): TUI task board

Add a cross-project task board to the TUI (T / palette): a roster fetched
from GET /tasks with vim-style nav, refresh, and an enter-to-open detail
pane (goal, criteria, paths, links, notes) built from the list payload — no
extra fetch. Server: TaskService.listAll() and a project-optional GET /tasks
(scoped with ?project=, whole board without; recent-first).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/tui-go/internal/app/overlays.go

@@ -125,6 +125,8 @@ func (m Model) renderOverlay(base string) string {
 		return m.center(m.ideasModal())
 	case OverlaySessions:
 		return m.center(m.sessionsModal())
+	case OverlayTasks:
+		return m.center(m.tasksModal())
 	case OverlayFiles:
 		return m.center(m.filesModal())
 	case OverlayStatusbar:
@@ -846,6 +848,7 @@ func (m Model) helpBody() []string {
 		{"e", "event inspector (/ to filter)"},
 		{"t / v", "tools / artifacts"},
 		{"S / R", "session stats / resume past sessions"},
+		{"T", "task board (across projects)"},
 		{"m / g", "swap model / edit config"},
 		{"G / I", "grants / idea board"},
 	})