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>

README.md

@@ -52,6 +52,7 @@ ssr restore [Brewfile]   Restore the machine; Brewfile defaults to the cloud cop
 ssr update [--dry-run]   Pull latest ssr code, refresh launchd if needed
 ssr schedule on|off      Enable/disable the daily launchd job
 ssr config show|edit     Show or edit the active config
+ssr completions install  Install zsh/bash/fish tab-completion
 ssr status               Show config, cloud target, last sync, schedule state
 ssr version              Print version
 ```
@@ -124,6 +125,31 @@ Config lives in `~/.config/ssr/ssr.conf` (override with `--config` or `$SSR_CONF
 
 ---
 
+## Shell completions
+
+`ssr install` offers to install completions automatically. To do it manually later:
+
+```bash
+ssr completions install                # detects $SHELL
+ssr completions install --shell all    # zsh + bash + fish
+ssr completions status                 # show what's wired up
+ssr completions uninstall --shell all
+```
+
+Targets (user-scoped, no sudo):
+
+| Shell | Path |
+|-------|------|
+| zsh   | `~/.zsh/completions/_ssr` |
+| bash  | `~/.local/share/bash-completion/completions/ssr` |
+| fish  | `~/.config/fish/completions/ssr.fish` |
+
+For zsh, ensure `~/.zsh/completions` is on `$fpath` (the installer prints the snippet for `~/.zshrc` if it isn't already). For bash, you need [`bash-completion@2`](https://github.com/scop/bash-completion) sourced from your shell rc — Homebrew install path is also printed. Fish needs no extra setup.
+
+After installation, open a fresh shell or `exec $SHELL` and `ssr <TAB>` lists the available commands.
+
+---
+
 ## Scheduled sync
 
 ```bash
@@ -184,6 +210,7 @@ ssr update --dry-run  # show what would change, don't pull
 │   ├── ssr-restore         # Rebuild a Mac from the backup
 │   ├── ssr-schedule        # launchd on/off/status
 │   ├── ssr-config          # show/edit/path
+│   ├── ssr-completions     # install zsh/bash/fish completions
 │   └── ssr-status          # state summary
 ├── lib/
 │   ├── common.sh           # Logging, validation, config loader
@@ -194,6 +221,10 @@ ssr update --dry-run  # show what would change, don't pull
 ├── config/
 │   ├── ssr.conf.example    # Reference user config
 │   └── macos-defaults.sh   # Default macOS preferences
+├── completions/
+│   ├── _ssr                # zsh
+│   ├── ssr.bash            # bash
+│   └── ssr.fish            # fish
 ├── launchd/
 │   └── de.surke.ssr.sync.plist  # Template for the scheduled agent
 ├── VERSION