fix: forget saved credentials when a synced/imported profile is retargeted; --answer needs --screenshot

- Saved credentials are keyed by profile id. A sync peer (or a crafted
  import using 'overwrite') could keep an existing profile's id but point
  it at another host, user, GitHub API URL, RD Gateway or jump host, and
  the saved password/token went there on the next connect without a
  prompt. Such a change now forgets that profile's saved credentials
  (audited), so the user is asked again.
- --answer / NEXUS_ANSWERS auto-fill every credential prompt and --answer
  is visible in the process list; they're now refused unless --screenshot
  is given (they exist for screenshot automation).
- Docs: IRC passwords are only sent over TLS/tunnel/loopback; sync's
  retarget behaviour; the --answer restriction.

rust/app/src/main.rs

@@ -99,8 +99,11 @@ fn main() -> eframe::Result {
         // SAFETY: still single-threaded here; no other thread can be reading the environment.
         unsafe { std::env::remove_var("NEXUS_ANSWERS") };
     }
+    // They answer every credential prompt in order (and --answer is visible in the process list),
+    // so they're only honoured for screenshot runs.
     if !opts.answers.is_empty() && opts.screenshot.is_none() {
-        eprintln!("warning: --answer/NEXUS_ANSWERS auto-fill credential prompts; they are meant for screenshot automation only");
+        eprintln!("--answer/NEXUS_ANSWERS only work together with --screenshot (they auto-fill credential prompts)");
+        std::process::exit(2);
     }
     let config = match data_dir {
         Some(d) => {