Retire vestigial feed contracts + protocol surface (ADR-0052 §5 follow-up)

[os-zhuang]

Goal

Complete the service-feed retirement (ADR-0052 §5, service deleted in #1955) by removing the now-vestigial feed contracts + protocol surface. sys_comment/sys_activity is the canonical record-collaboration/timeline backend; the feed REST protocol was never mounted (404) and has no implementation.

This is a multi-package refactor (mapped via build-gating). Do it as a focused PR when the repo is quiet — it touches the core IServerProtocol and spec, which everything depends on.

⚠️ Keep — do NOT delete

packages/spec/src/data/feed.zod.ts — its FeedItemType and FeedFilterMode are live, consumed by the record:chatter UI component (packages/spec/src/ui/component.zod.ts → RecordActivityProps/RecordChatterProps). Only the retired-backend types are dead. (Consider later renaming feed→activity, but keep for now.)

Scope (blast radius, build-verified)

spec

• Delete contracts/feed-service.ts (IFeedService), api/feed-api.zod.ts, data/subscription.zod.ts + their *.test.ts.
• Remove their barrel exports: contracts/index.ts, api/index.ts, data/index.ts (keep feed.zod).
• api/protocol.zod.ts — remove the 4 feed regions: (1) the import type { … } from './feed-api.zod' block, (2) the // Feed Operations z.function() stubs, (3) the // Feed Types export type { … } re-export block, (4) the // Feed (optional) methods on the IServerProtocol interface.

objectql

• src/protocol.ts — remove the getFeedService?: () => IFeedService constructor param, requireFeedService(), and the feed method implementations; update constructor call sites that pass getFeedService.
• Delete src/protocol-feed.test.ts.

client (public SDK breaking change — but the methods called the never-mounted feed REST, so already dead)

• src/index.ts — remove the two feed-api type-import blocks (~lines 69-82 and ~3497-3510) and the entire feed = { … } accessor (~150 lines, list/create/update/delete/reactions/pin/star/search/changelog/subscribe).

Verify

pnpm --filter @objectstack/spec build → objectql → client → rest, all green. Then pnpm --filter @objectstack/plugin-audit test etc.

Context

Spun out of the platform-activity work (#1948 trackHistory, #1952 milestones, #1954 sys_attachment→storage, #1955 retire service-feed, #1957 ActivityPointer). I attempted this cleanup but backed out cleanly because (a) the ripple was large and (b) the repo was being concurrently branch-switched mid-refactor — unsafe to push then.