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/help.go

@@ -0,0 +1,59 @@
+package tui
+
+import "strings"
+
+func renderHelp(width, height int) string {
+	sections := []struct {
+		title string
+		binds [][2]string
+	}{
+		{"Navigation", [][2]string{
+			{"j/k ↑/↓", "move cursor"},
+			{"g/G home/end", "top / bottom"},
+			{"pgup/pgdn", "page up / down"},
+			{"enter", "view detail"},
+			{"esc", "back / cancel"},
+		}},
+		{"Actions", [][2]string{
+			{"a", "add entity"},
+			{"d", "delete (with confirm)"},
+			{"x", "toggle todo completion"},
+			{"!", "toggle pin"},
+			{"#", "filter by tag"},
+		}},
+		{"Detail View", [][2]string{
+			{"p", "promote to card"},
+			{"D", "demote to fluid"},
+			{"c", "copy to clipboard"},
+			{"e", "edit in $EDITOR"},
+			{"!", "toggle pin"},
+		}},
+		{"Global", [][2]string{
+			{"?", "toggle help"},
+			{"q / ctrl+c", "quit"},
+		}},
+	}
+
+	var b strings.Builder
+	b.WriteString(detailHeaderStyle.Render("keybindings"))
+	b.WriteString("\n\n")
+
+	for _, s := range sections {
+		b.WriteString(titleStyle.Render(s.title))
+		b.WriteString("\n")
+		for _, bind := range s.binds {
+			key := helpKeyStyle.Render(bind[0])
+			desc := helpDescStyle.Render(bind[1])
+			b.WriteString("  " + key + "  " + desc + "\n")
+		}
+		b.WriteString("\n")
+	}
+
+	b.WriteString(helpStyle.Render("press ? or esc to close"))
+
+	lines := strings.Split(b.String(), "\n")
+	if len(lines) > height {
+		lines = lines[:height]
+	}
+	return strings.Join(lines, "\n")
+}