feat(tui): add status bar, help overlay, tag filter, and entity actions

Status bar with entity count and context-sensitive key hints. Help
overlay via ? key. Tag filter via # with cursor-navigable tag list.
Todo toggle (x), pin (!), promote (p), demote (D), copy (c), edit (e)
via $EDITOR. Delete confirmation with 3s timeout. Date-grouped list
with completed todo and pinned indicators. Esc clears active tag filter.

Adds CompletedAt/ClearCompleted to EntityUpdate for todo toggling.

internal/tui/statusbar.go

@@ -0,0 +1,46 @@
+package tui
+
+import (
+	"fmt"
+
+	"github.com/charmbracelet/lipgloss"
+)
+
+func renderStatusBar(m model, width int) string {
+	left := countText(m)
+	right := contextHints(m)
+
+	leftRendered := statusStyle.Render(left)
+	rightRendered := helpStyle.Render(right)
+
+	gap := width - lipgloss.Width(leftRendered) - lipgloss.Width(rightRendered)
+	if gap < 0 {
+		gap = 0
+	}
+
+	pad := lipgloss.NewStyle().Width(gap).Render("")
+	return leftRendered + pad + rightRendered
+}
+
+func countText(m model) string {
+	total := len(m.list.entities)
+	if m.filterTag != "" {
+		return fmt.Sprintf("%d entities #%s", total, m.filterTag)
+	}
+	return fmt.Sprintf("%d entities", total)
+}
+
+func contextHints(m model) string {
+	switch m.state {
+	case stateDetail:
+		return "p:promote D:demote c:copy e:edit !:pin esc:back"
+	case stateInput:
+		return "enter:submit esc:cancel"
+	case stateTagFilter:
+		return "j/k:nav enter:select esc:cancel"
+	case stateConfirm:
+		return "y:confirm n:cancel"
+	default:
+		return "a:add d:del x:todo #:filter ?:help q:quit"
+	}
+}