Retire residual feed discovery/dispatcher surface (ADR-0052 §5, follow-up to #1959)

[os-zhuang]

Background

#1959 retired the vestigial feed contracts + protocol + client surface (ADR-0052 §5): IFeedService, feed-api.zod, data/subscription.zod, the FeedProtocol slice of protocol.zod, the getFeedService plumbing in metadata-protocol, and the client.feed.* accessor. service-feed's runtime was already deleted in #1955.

A small discovery/dispatcher feed surface was deliberately left out of #1959 to keep that PR focused, because it is entangled with the comments capability. This issue tracks finishing it.

Scope

• packages/spec/src/api/discovery.zod.ts
  • routes.feed (ApiRoutes) — the feed: '/api/v1/feed' route hint. Removing it also lets packages/client/src/index.ts drop the feed route constant it currently keeps only to satisfy the Record<ApiRouteType, string> type (see the NOTE comment added there in Retire vestigial feed contracts + protocol surface (ADR-0052 §5 follow-up) #1959).
  • WellKnownCapabilitiesSchema.feed — the boolean capability. Blocker to untangle: comments currently derives from the same deleted service.
• packages/spec/src/api/dispatcher.zod.ts
  • the { prefix: '/api/v1/feed', service: 'data' } dispatcher entry.
• packages/metadata-protocol/src/protocol.ts getDiscovery()
  • feed: registeredServices.has('feed') — since service-feed no longer exists this is permanently false; and
  • comments: registeredServices.has('feed') — this also keys off the deleted 'feed' service, so comments is now permanently false too. That is a real bug: sys_comment is the canonical comments backend, so the comments capability should reflect that, not the retired feed service. Decide where comments should hang (e.g. always-on, or keyed off the data/sys_comment availability) as part of this cleanup.
• Tests to update: packages/spec/src/api/discovery.test.ts, packages/spec/src/api/dispatcher.test.ts, packages/client/src/client.test.ts (capabilities.feed).

Notes

• Removing WellKnownCapabilities.feed is a breaking change to the discovery response shape → needs a changeset with a FROM → TO note (capabilities.feed → capabilities.comments / the services registry).
• Regenerate spec artifacts (gen:schema may need a manifest key removal for the discovery schema, gen:api-surface, gen:docs) as in Retire vestigial feed contracts + protocol surface (ADR-0052 §5 follow-up) #1959.