feat: add calendar visibility picker

Derive session calendars from event metadata and apply one visibility contract to month rendering, appointment focus, and search without filtering persistence writes.

Add keyboard picker coverage, full-model save regression tests, documentation, warning-clean builds, sanitizer verification, and live PTY terminal restoration checks.

src/main.cpp

@@ -57,6 +57,7 @@ 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"
+        "      c toggles calendar visibility, / searches appointments,\n"
         "      t jumps to today, ? toggles help, q quits.\n";
 }