Replace bare backticks with cli inline markup across all messaging calls

- {.arg} for argument names in stop_/warning_/message_ calls
- {.cls} after "of class" text in format_class() and elsewhere
- {.fun} for function names (replaces `fn()` pattern)
- {.pkg} for tidyverse package names (dplyr, ggplot2)
- {.code} for code literals (TRUE, FALSE, expressions)
- Rewrite print.ab: use cli named-vector with * bullets and code
  highlighting when cli >= 3.0.0; keep plain-text fallback otherwise
- Fix typo in as.sir(): "of must be" -> "or must be"
- switch sir.R verbose notes from message() to message_()

https://claude.ai/code/session_01XHWLohiSTdZvCutwD7ag2b

R/get_episode.R

@@ -215,7 +215,7 @@ is_new_episode <- function(x, episode_days = NULL, case_free_days = NULL, ...) {
 
 exec_episode <- function(x, episode_days, case_free_days, ...) {
   stop_ifnot(is.null(episode_days) || is.null(case_free_days),
-    "either argument `episode_days` or argument `case_free_days` must be set.",
+    "either argument {.arg episode_days} or argument {.arg case_free_days} must be set.",
     call = -2
   )