Plugin seam for #621 (LWS), under #206.

Problem

LWS requires Link: <storage>; rel="https://www.w3.org/ns/lws#storage" on every response from the storage, plus up, type and linkset relations on resources. Plugins get a scoped Fastify instance (api.fastify), so an onSend hook registered there only fires under the plugin's prefix — never on /alice/foo.

Discovery links are the canonical "a plugin wants to be visible from pod resources" need (ActivityPub rel="alternate" to an actor, Nostr rel="nostr", remoteStorage rel="remotestorage" would all use it), so this is worth a general seam rather than LWS-specific code.

Proposal

api.headers.link((request, { url, isContainer, isRoot, pod }) => {
  // return an array of { href, rel, attrs? } or nothing
  return [{ href: pod.storageUrl, rel: 'https://www.w3.org/ns/lws#storage' }];
});

• Called from the existing header builder in src/ldp/headers.js (getLinkHeader) for every LDP response, so output is merged into the one Link header rather than appended as a second one.
• Also exposed under the plugin's own prefix for symmetry.
• Headers only — a plugin cannot change status or body here. Content negotiation for application/lws+json / application/lws+cid is core work (Track: LWS protocol conformance (Oct 2026 drafts) — core vs plugin split, Touchstone as acceptance test #621), not this seam.

Prior art: LwsLinkMetadataWriter in jeswr/LinkedWebStorage.