feat(tasks): task search across REST, tool, and TUI

Ranked exact/substring search: TaskSearch (all terms AND, ranked title >
key > goal > criteria > notes) over one project or the whole board via
TaskService.search. Surfaced as GET /tasks?q=, a read-only task_search tool
for agents, and a `/` filter in the TUI board.

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

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

@@ -326,6 +326,8 @@ type Model struct {
 	taskListErr      string
 	taskDetail       bool
 	taskDetailScroll int
+	taskFilter       string // `/` substring narrow over id/title/goal
+	taskFilterTyping bool
 
 	// command palette
 	paletteFilter string