update README and zsh completions for profiles feature

- Document hush profiles subcommands and --profile flag in command table
- Add Profiles section with file format, creation flow, and hotkey example
- Expand configuration table with fireworks_api_key, provider fields, system_prompt
- Add profile name completion via _hush_profiles helper in zsh completion
- Add --profile/-p completion for toggle and stop commands
- Add profiles subcommand completion

completions/_hush

@@ -20,6 +20,7 @@ _hush() {
                 'latency-test:Run a full STT+LLM round-trip latency test'
                 'setup:Interactive configuration wizard'
                 'show:Display current configuration'
+                'profiles:Manage configuration profiles'
             )
             _describe 'command' commands
             ;;
@@ -30,9 +31,31 @@ _hush() {
                         '--plain[Show plain key=value pairs instead of a table]' \
                         '--json[Show raw JSON output]'
                     ;;
+                toggle|stop)
+                    _arguments \
+                        '--profile[Profile name to apply when processing]:profile:_hush_profiles' \
+                        '-p[Profile name to apply when processing]:profile:_hush_profiles'
+                    ;;
+                profiles)
+                    local subcommands=(
+                        'list:List all available profiles'
+                        'get:Print the contents of a profile'
+                        'new:Create a new profile'
+                        'edit:Open a profile in $EDITOR'
+                    )
+                    _describe 'profiles subcommand' subcommands
+                    ;;
             esac
             ;;
     esac
 }
 
+_hush_profiles() {
+    local profiles_dir="${HOME}/.config/hush/profiles"
+    if [[ -d "$profiles_dir" ]]; then
+        local profiles=("${profiles_dir}"/*(N:t))
+        _describe 'profile' profiles
+    fi
+}
+
 _hush "$@"