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/model.go

@@ -57,6 +57,7 @@ const (
 	OverlayStats
 	OverlayIdeas
 	OverlaySessions
+	OverlayTasks
 	OverlayFiles
 	OverlayStatusbar
 	OverlayGrants
@@ -317,6 +318,15 @@ type Model struct {
 	sessionListLoading bool
 	sessionListErr     string
 
+	// task board (OverlayTasks) — all tasks across projects, fetched over HTTP from
+	// GET /tasks. taskDetail toggles the per-task detail pane (built from the list payload).
+	taskList         []TaskSummary
+	taskListIndex    int
+	taskListLoading  bool
+	taskListErr      string
+	taskDetail       bool
+	taskDetailScroll int
+
 	// command palette
 	paletteFilter string
 	paletteIndex  int