Export arbitrary WordPress sites as replayable Playground blueprint bundles

## Problem
Workflow Bench needs every website-generation adapter to leave behind a durable, replayable WordPress site, not just logs or copied files. Some generated sites come from WP Codebox runtimes, but others come from external/local WordPress sites such as Studio-managed sites. Those external generated sites need a generic export path into a replayable Playground/WP Codebox bundle.

Existing issue #220 covered replayable runtime snapshot bundles for WP Codebox-owned runtimes. This issue extends the requirement to arbitrary generated WordPress sites that were not originally created inside a Codebox runtime.

## Desired capability
Provide a generic exporter that can take a generated WordPress site source and produce a durable replay bundle.

Inputs could include:
- a WordPress site path on disk
- a WP Codebox runtime
- a Studio-generated local site directory
- a generated artifact bundle with WordPress files/state

Output should be a replayable bundle, for example:
- `manifest.json`
- `blueprint.json`
- `files/` containing themes/plugins/uploads/import payloads as needed
- site-state export where needed for pages/options/terms/templates/media
- replay limitations/diagnostics
- SHA-256 verified artifact refs

## Acceptance criteria
- A generated WordPress site outside Codebox can be exported into a `wp-codebox/replayable-wordpress-site/v1` style bundle.
- The bundle can be replayed in WP Playground / WP Codebox without relying on the original local site path.
- The bundle preserves enough state to inspect generated pages, templates, active theme/plugins, media/uploads, and relevant options.
- The manifest clearly distinguishes fully replayable state from partial/limited captures.
- The command is policy-controlled and does not implicitly export private sites.
- Workflow Bench can consume the emitted bundle as a durable generated-site artifact.

## Non-goals
- Do not score generation quality here.
- Do not add Studio-specific behavior to WP Codebox; Studio should be just one consumer/source.
- Do not require changes in `Automattic/studio` for the initial capability.