Modernise messaging infrastructure with cli support

Rewrites message_(), warning_(), stop_() to use cli::cli_inform(),
cli::cli_warn(), and cli::cli_abort() when the cli package is available,
with a fully functional plain-text fallback for environments without cli.

Key changes:
- New cli_to_plain() helper converts cli inline markup ({.fun}, {.arg},
  {.val}, {.field}, {.cls}, {.pkg}, {.href}, {.url}, etc.) to readable
  plain-text equivalents for the non-cli fallback path
- word_wrap() simplified: drops add_fn, ANSI re-index algorithm, RStudio
  link injection, and operator spacing hack; returns pasted input unchanged
  when cli is available
- stop_() no longer references AMR_env$cli_abort; uses pkg_is_available()
  directly; passes sys.call() objects to cli::cli_abort() call= argument
- Removed add_fn parameter from message_(), warning_(), and word_wrap()
- All call sites across R/ updated: add_fn arguments removed, some paste0-
  based string construction converted to cli glue syntax ({.fun as.mo},
  {.arg col_mo}, {n} results, etc.)
- cli already listed in Suggests; no DESCRIPTION dependency changes needed

https://claude.ai/code/session_01XHWLohiSTdZvCutwD7ag2b

R/zzz.R

@@ -118,8 +118,7 @@ AMR_env$cross_icon <- if (isTRUE(base::l10n_info()$`UTF-8`)) "\u00d7" else "x"
   if (interactive() && is.null(getOption("AMR_guideline"))) {
     packageStartupMessage(
       word_wrap(
-        "Assuming ", AMR::clinical_breakpoints$guideline[1], " as the default AMR guideline, see `?AMR-options` to change this.",
-        add_fn = NULL
+        "Assuming ", AMR::clinical_breakpoints$guideline[1], " as the default AMR guideline, see `?AMR-options` to change this."
       )
     )
   }