Feature: Add wp cbt save CLI command

Part of #828 (which fulfills #688). Depends on the CBT_Theme_Save service refactor tracked in #828.

Goal

Expose the CBT_Theme_Save service as a WP-CLI subcommand so developers can persist Editor changes to disk from the terminal.

Scope

New file: includes/cli/class-cbt-cli-command.php

New class: CBT_CLI_Command extending WP_CLI_Command, with a save method.

Registration (in includes/class-create-block-theme.php):

if ( defined( 'WP_CLI' ) && WP_CLI ) {
    require_once plugin_dir_path( __DIR__ ) . 'includes/cli/class-cbt-cli-command.php';
    WP_CLI::add_command( 'cbt', 'CBT_CLI_Command' );
}

CLI surface

wp cbt save [--fonts] [--templates] [--styles] [--patterns]
            [--all] [--only-modified-templates] [--dry-run]

• Flags are kebab-case publicly; the command translates internally to the saveFonts / saveTemplates / etc. keys the service expects
• --all is a convenience for "save everything"
• --only-modified-templates maps to processOnlySavedTemplates
• --dry-run reports what would change without writing

Required behavior

• Guard with wp_is_block_theme() — WP_CLI::error() if not, mirroring the plugin UI gate documented in AGENTS.md
• WP_CLI::error() if no save flag is passed (no silent no-op success)
• All user-facing strings i18n'd via __( ..., 'create-block-theme' )
• PHPDoc with shortdesc, full ## OPTIONS, and at least two ## EXAMPLES
• Translate WP_Error from the service into WP_CLI::error()

Tests

PHPUnit tests against the command class (the service itself is covered in its own issue):

• Flag normalization (kebab-case → service options)
• Guard fires on non-block themes
• Empty invocation produces an error

Out of scope

• wp cbt export — separate issue tracked in #828
• Behat features — not in the plugin's infrastructure today
• Any other CBT operation as a CLI command

Acceptance

• Manual smoke test: install plugin in a Studio site, run each flag combination against a block theme
• wp help cbt save displays the documented options and examples
• lint:php, test:php, test:unit all green

Start with AGENTS.md, includes/class-create-block-theme.php, and the CBT_Theme_Save service refactor in #828; then add PHPUnit coverage for flag normalization and the non-block-theme and empty-invocation guards in the new command class. Done means the command is registered, its help shows the options and examples, manual flag combinations work on a block theme, and lint:php, test:php, and test:unit pass.