Add Substack migration support

[joeboydston]

Summary

• New scripts/substack/discover.js — inventories a Substack publication via its undocumented public API (/api/v1/archive). No browser dependency.
• New scripts/substack/extract.js — extracts all content via API, with --csv-export flag for full paid post content from Substack's official export. Unwraps substackcdn.com CDN wrapper URLs to download original images.
• New prompts/substack.md — user-facing migration prompt covering content, subscribers, paid tiers, and redirects.
• Updated scripts/import.js — adds --self-hosted flag for non-WordPress.com sites, fixes media upload Content-Type handling, improves image URL replacement to handle Substack CDN wrapper URLs. Wix import behavior unchanged.
• Updated AGENTS.md, README.md, DISCOVERIES.md with Substack documentation.

How I found it

Built and tested as a new platform contribution. Substack's public API endpoints (/api/v1/archive, /api/v1/posts/<slug>) return rich JSON without authentication, making extraction fast and browser-free.

Tested against

• Real Substack publication (public, all-free content, 265 posts)
• Scripts run without errors
• Full end-to-end import to a WordPress staging site
• Images correctly replaced from Substack CDN URLs to WordPress media URLs
• Posts created as drafts with correct titles, dates, and content

Discovery log entry added to DISCOVERIES.md

• Yes

🤖 Generated with Claude Code

[simison]

Really cool idea to add Substack!

[simison]

We have APIs for doing both paid and free subscriber imports to WP.com and Jetpack sites, including the Stripe account switch, so those should first be exposed via MCP and then connected here.

Cc @Automattic/loop team who's looking after subscriber importer.

[simison]

Some potential problems to solve, not necessarily in this PR:

• Videos and podcasts are not included in substack exports and need to be scraped separately. For paid videos needs authenticating or maybe access to sites media management?
• Lots of "subscribe!" nudges in post content; could be removed, or replaced e.g. with MailPoet forms or Jetpack Subscription blocks. I'd imagine intuitively AI would use button or link blocks otherwise but there's no point if they aren't actually functional.
• Paywall markers in paid posts likely needs guidance which plugin/block to use; Jetpack Newsletters has a paywall block!

[robertbpugh]

Nice work. We've done a lot of Substack migrations on the WP.com side -- a few things that always come up:

• Paid subscriber imports are tough. Stripe is the source of truth, not the Substack export. Comp/gift subs don't come through at all since they're not in Stripe.
• Subscription dates get lost, everyone shows the import date, not their original sub date.
• There are REST endpoints for both free and paid subscriber import, including Stripe plan mapping. We're working on subscription date preservation now, and wrapping these for MCP would close the biggest gap, letting the agent handle full subscriber migration end-to-end without manual CSV steps.