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-install

@@ -44,4 +44,9 @@ if ssr::confirm "Enable daily scheduled sync (launchd)?" 30; then
     "$SSR_ROOT/bin/ssr-schedule" on
 fi
 
+# Offer to install shell completions for the detected shell.
+if ssr::confirm "Install shell completions for $(basename "${SHELL:-unknown}")?" 30; then
+    "$SSR_ROOT/bin/ssr-completions" install
+fi
+
 ssr::ok "Install complete. Run:  ssr status"