Add shell auto-completion for zsh, bash, and fish

- completions/_ssr            zsh #compdef completion using _arguments/_values
- completions/ssr.bash        bash complete -F function (works with bash-completion@2)
- completions/ssr.fish        fish completions with __ssr_needs_subcmd predicates
- bin/ssr-completions         install/uninstall/status subcommand
                              - auto-detects $SHELL, --shell zsh|bash|fish|all
                              - symlinks into user-scoped paths (no sudo):
                                ~/.zsh/completions/_ssr
                                ~/.local/share/bash-completion/completions/ssr
                                ~/.config/fish/completions/ssr.fish
                              - prints fpath/bash-completion setup hints if not configured
- bin/ssr                     dispatcher knows `completions`; help line added
- bin/ssr-install             offers completion install after schedule prompt
- README                      Shell completions section, command list, layout entry

Verified:
- zsh -n  on _ssr        OK; autoload registers function
- bash -n on ssr.bash    OK; `complete -p ssr` returns the registration after source
- end-to-end install/status/uninstall against an ephemeral $HOME

Co-authored-by: Cursor <cursoragent@cursor.com>

bin/ssr

@@ -32,6 +32,7 @@ Commands:
   update [--dry-run]   Pull latest ssr code, refresh launchd if needed.
   schedule on|off      Enable/disable the scheduled launchd sync job.
   config show|edit     Print or edit the active config file.
+  completions          Install or remove shell completions (zsh/bash/fish).
   status               Show last sync time, cloud target, and scheduled state.
   version              Print version.
 
@@ -61,7 +62,7 @@ done
 
 cmd="$1"; shift
 case "$cmd" in
-    install|sync|restore|update|schedule|config|status)
+    install|sync|restore|update|schedule|config|completions|status)
         target="$SSR_BIN_DIR/ssr-$cmd"
         [[ -x "$target" ]] || ssr::die "Subcommand not executable: $target"
         exec "$target" "$@"