Consolidate public URL imports behind one resumable contract

[chubes4]

Problem

static-site-importer/import-url currently exposes three execution paths through opaque provider_args: single-page fetch, monolithic whole-site collection, and resumable batch import. A WordPress.com UI caller would need to know internal controls such as collect_site and batch_pages, and the paths can drift in collection, compilation, quality, finalization, and reporting behavior.

Direction

Make the resumable URL batch engine the canonical built-in public URL import path for sites of every size. The public ability accepts the source URL and import intent; SSI owns collection mode, batch size, invocation budget, limits, continuation identity, and progress. Provider-supplied artifacts must continue through the canonical artifact importer.

Acceptance criteria

• One static-site-importer/import-url contract supports WordPress.com UI callers without collect_site, batch_pages, or runtime tuning.
• Built-in public URL imports use one resumable implementation for one page or hundreds.
• Continuation responses retain one stable import identity and deterministic progress.
• Existing provider override behavior remains supported.
• Direct and batched behavior parity is covered for collection, import options, final activation, reports, quality, and idempotent replay.
• CLI remains a projection of the ability rather than a separate policy owner.

Related: #808. Runtime evidence is tracked by Automattic/wp-codebox#2212 and WordPress Playground PR WordPress/wordpress-playground#4172.

AI assistance

OpenAI gpt-5.6-sol via OpenCode was used to reproduce the split-path behavior, inspect the runtime and ability boundaries, and draft this issue. Chris Huber remains responsible for the issue and implementation.