feat: add bounded agenda view

Add a read-only 42-day occurrence agenda with keyboard navigation, six-week paging, exact return-to-grid focus, search and calendar overlays, and bounded recurrence expansion.

Keep agenda state outside persistence, document month-only mutations, and cover empty windows, filtering, overlays, navigation, rendering, and terminal restoration.

src/main.cpp

@@ -57,6 +57,8 @@ void print_help(std::ostream& output)
         "      Tab/Shift-Tab focus appointments, Enter reads, Esc returns,\n"
         "      a adds, e edits, d deletes; Ctrl-S saves inside the editor,\n"
         "      u undoes the last change, Ctrl-R redoes it,\n"
+        "      g opens the 42-day agenda; arrows/jk/Tab choose, Enter jumps,\n"
+        "      PageUp/PageDown or p/n shift its window, Esc/g returns,\n"
         "      c toggles calendar visibility, / searches appointments,\n"
         "      t jumps to today, ? toggles help, q quits.\n";
 }
@@ -159,6 +161,12 @@ int print_frame(std::span<const nocal::Event> events, bool no_color)
         .search_result_index = 0,
         .notification = {},
         .notification_is_error = false,
+        .show_calendar_picker = false,
+        .calendar_index = 0,
+        .show_agenda = false,
+        .agenda_start_day = day,
+        .agenda_items = {},
+        .agenda_index = 0,
     };
     std::cout << nocal::tui::render_month(events, state) << '\n';
     return std::cout ? EXIT_SUCCESS : EXIT_FAILURE;