Restore Webmention forms on block themes and hide classic-only checkbox

[pfefferle]

Release archives excluded build/form/block.json through the /build/*/block.json export-ignore rule. Without that metadata, WordPress cannot register or automatically insert the Webmention Form block, while block themes skip the classic form. This leaves no Webmention form even when the checkbox is enabled. On notiz.blog, the metadata URL returns 404 while the form JavaScript returns 200.

Include the form block metadata in release archives and only skip the classic form when the form block is registered. Other build-entry metadata stays excluded.

Hide the "Show a Webmention form" checkbox only on block themes and ignore its saved value when using Block Hooks. Classic themes retain the checkbox and its behavior; the custom help-text field remains. The form block can be moved or removed in the Site Editor.

Validation:

• 13 form tests pass, including actual hook insertion and rendering with a block theme and the legacy setting disabled, plus the classic fallback when block registration is missing.
• git archive HEAD build/form includes build/form/block.json.
• PHP coding checks and git diff --check pass.
• PHPUnit reports the existing configuration warning about the source element under PHPUnit 9.

Follow-up to #667.