feat: add netssh uninstall

Remove everything netssh and install.sh set up, also leftovers of
older versions:

- netssh blocks and unmarked legacy lines in bash, zsh, and fish
  profiles
- completion files for bash, zsh (incl. the oh-my-zsh cache), fish,
  and the command alias; files not generated by netssh are kept
- the cache directory with hosts, aliases, and per-instance caches
- the config file and local notes; the config is first copied to a
  private netssh.yaml.backup-<timestamp> because it holds API tokens
- the netssh binary; for protected paths the sudo command is printed

--keep-config keeps config and notes, --dry-run only lists the plan,
and --yes skips the confirmation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

install.sh

@@ -258,3 +258,4 @@ for kind in "${COMPLETIONS[@]}"; do
     info "zsh: if <Tab> still shows files, run: rm -f $(quote_sh "$ZSH_DIR")/.zcompdump* && exec zsh"
   fi
 done
+info "Remove netssh again with: netssh uninstall"