Feature: Add wp cbt export CLI command

[bph]

Part of #828 (which fulfills #688). Depends on the CBT_Theme_Export service refactor tracked in #828, and reuses the CBT_CLI_Command class introduced for wp cbt save.

Goal

Expose the CBT_Theme_Export service as a WP-CLI subcommand so developers can produce a theme zip from the terminal.

Scope

Add an export method to CBT_CLI_Command. No new file.

CLI surface

wp cbt export <output-path> [--include-images]
                            [--export-ready]
                            [--translate-ready]

• Positional <output-path> — required, must be writable
• Flag names align with the existing CBT export option labels
• Progress reporting via WP_CLI\Utils\make_progress_bar() for image bundling

Required behavior

• wp_is_block_theme() guard — WP_CLI::error() if not
• Reject <output-path> if the parent directory does not exist or is not writable, with a clear WP_CLI::error() message
• All user-facing strings i18n'd
• PHPDoc with shortdesc, ## OPTIONS, ## EXAMPLES

Tests

• Output path validation (non-existent dir, non-writable dir, dir vs file)
• Flag normalization
• Block theme guard

Out of scope

• --directory (unzipped) output — possible follow-up
• Variation / clone / blank export — possible follow-up
• Custom theme metadata overrides at export time

Acceptance

• Manual smoke test: wp cbt export /tmp/my-theme.zip --include-images against a real block theme in Studio
• wp help cbt export displays the documented options and examples
• lint:php, test:php, test:unit all green