Document Link Pay Tokens and checkout authorization for Link cards

[rgarcia]

Summary

Link cards now authorize at the checkout page: authorize takes browser_id and page_url, and KERNEL inspects the checkout to deliver either a merchant-bound Link Pay Token (supported Stripe checkouts) or a virtual card. These pages still described the previous flow.

• Link by Stripe: lifecycle and authorize examples (SDK and CLI) pass browser_id/page_url; new sections compare Link Pay Tokens and virtual cards and list authorization errors with recovery steps; fill is split into a Link Pay Token fill (no fields) and a virtual card fill; amount limits are 500000 overall and 50000 for a virtual card; card updates work until the card is authorized at a checkout, then return 409.
• Enable payments in a browser agent: the Link tab and the copy-paste CLI prompt create the card at checkout, authorize with --browser-id/--page-url, and follow the advertised fill description.
• Wallets overview, payments overview, fill: comparison table and coverage note mention both credentials; fill notes that a Link Pay Token takes no field bindings.

Replaces #630, which documented an earlier design.

Validation

• mint broken-links: no broken links.
• mint dev: all five edited pages render.

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[bmsaadat]

thanks, this lines up with the API, CLI v0.49.0 and SDK 0.126.0. requesting changes for two things:

1. over-limit advice (stripe-link.mdx:514): "update the card to a smaller amount or tell the user this checkout can't be paid with link" should match the API and CLI: delete the card and tell the user this checkout can't be paid with a link virtual card. lowering the amount breaks the confirmed purchase, and only the virtual card is ruled out.
2. stale wording on the wallets overview (overview.mdx:8, :52): both still say link only issues single-use cards, which contradicts the updated table on the same page.

nits (optional):

• browser_unavailable also covers a busy browser (another fill holds it) and deleted, unleased or other-project browsers.
• "connected parent wallet" (:523) only applies to pay token fills, not link virtual cards.
• :567: there's also a merchant check after the token is fetched, so "nothing is passed to the page" is safer than "before any token is retrieved".
• the TypeScript authorize example doesn't type-check on SDK 0.126.0 (pre-existing): the operation response union includes the fill result, so narrow on type === "card".

[rgarcia]

Addressed in 14366de:

1. Over-limit advice: now matches the API and CLI: delete the card and tell the user this checkout can't be paid with a Link virtual card, without lowering a confirmed purchase.
2. Wallets overview: both sentences now describe a Link Pay Token on supported Stripe checkouts or a single-use virtual card elsewhere.

Nits, all done: browser_unavailable row lists busy, unleased, deleted and other-project browsers; the connected-wallet requirement is scoped to Link Pay Token fills; the merchant-mismatch line now says nothing is passed to the page, with checks before retrieval and before passing; the TypeScript authorize example narrows on type === "card".

Also since your review: spec.expires_at was removed from Link cards (kernel/kernel#4836, now deployed), and the docs no longer mention it.