[Tracking] WP-CLI support: phased implementation

Tracking issue for adding WP-CLI support to Create Block Theme, originally requested in #688.

Background

Issue #688 proposes WP-CLI commands so developers can persist Editor changes to disk from the terminal — useful for git workflows and automation. @bacoords prototyped a wp cbt save command on a separate branch (https://github.com/bacoords/create-block-theme/tree/add/cli-export-command) but did not open a PR; the prototype acknowledges duplicated logic from the REST API.

This tracking issue proposes a four-PR sequence that ships the CLI as a thin layer over a properly extracted service layer, so REST and CLI share a single implementation.

Approach

For each of save and export:

1. Refactor first — extract the REST handler's orchestration into a service class (CBT_Theme_Save, CBT_Theme_Export) under includes/create-theme/. REST endpoint becomes a thin wrapper. Add PHPUnit coverage. No behavior change at the wire boundary.
2. Add the CLI command — register on the same service. Kebab-case flags, wp_is_block_theme() guard, ## EXAMPLES, i18n.

Doing the refactor first means the test coverage we need anyway lands in a small, reviewable PR before any new surface area is added.

Variation, clone, child-theme, blank-theme, and reset commands are deliberately out of scope for v1 and can be added in follow-ups once the pattern is established.

Sub-issues

• #829 — Extract CBT_Theme_Save service from rest_save_theme
• #830 — Add wp cbt save CLI command
• #831 — Extract CBT_Theme_Export service from rest_export_theme
• #832 — Add wp cbt export CLI command

Out of scope (potential follow-ups)

• Variation / clone / blank / child theme creation as CLI commands
• Behat feature tests (the plugin has no Behat infrastructure today)
• Promoting silent filesystem failures to WP_Error (separate concern from the refactor)

References

• Original feature request: #688
• Prototype branch: https://github.com/bacoords/create-block-theme/tree/add/cli-export-command
• WP-CLI Commands Cookbook: https://make.wordpress.org/cli/handbook/guides/commands-cookbook/

Open to discussion on scope, sequencing, and naming before any PRs are opened.

Start with sub-issue #829 and the REST handlers named in this tracking issue, rest_save_theme and rest_export_theme, under includes/create-theme/. Review the proposed sequence and PHPUnit coverage requirements; the tracking work is done when the relevant service extraction or CLI sub-issue is completed without taking the listed out-of-scope commands into v1.