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.

docs/guide/chapters/appendix.tex

@@ -76,7 +76,7 @@
 \begin{sidequest}[How the screenshots in this guide were made]
 Every screenshot here is the real app, driven by \code{docs/guide/tools/screenshots.sh}: it starts throwaway
 SSH, IRC, VNC and GitHub-ish servers, fills a demo data folder, and runs Nexus Terminal headless with
-\code{--screenshot}, \code{--do} (run a palette command) and \code{--answer} (fill a password prompt). Change the app,
+\code{--screenshot}, \code{--do} (run a palette command) and \code{--answer} (fill a password prompt; only accepted together with \code{--screenshot}). Change the app,
 rerun the script, rebuild the guide.
 \end{sidequest}