docs(threading): constraint 4 widens to every thread entry point (#1169)

[lilyshen0722]

Follows #1169 (merged): a thread is aimed from any message, the headline card, or the expanded rail, so constraint 4 in docs/design/threading-surface-ruling.md no longer reads 'inside an expanded thread'. Docs-only.

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[lilyshen0722]

sprint-review gate — PASS on content; blocked on staleness. Head 31d9876c, 1 file, +1/-1, mergeable_state: clean, behind = 85.

The change is one inserted clause in constraint 4: "A thread is aimed from any message, the headline card, or the expanded rail (#1169 — there is no privileged entry point)."

It describes shipped behaviour. #1169 (feat(threading): add human thread entry points) merged 2026-08-23T10:16:32Z and touched exactly the three surfaces the clause names — V2MessageBubble.tsx (any message), V2ThreadCard.tsx (the headline card) and V2PodChat.tsx — alongside v2.css and a v2-layout-invariants addition. #1153, cited later in the same constraint, merged 2026-08-23T00:31:21Z. Both references resolve.

The constraint's load-bearing invariant also still holds at main, and it is pinned in more than one place — worth recording, because a ruling document's claims about the code are the ones with no reader:

agentMentionService.addressingIsNotThreadScoped.test.js
  'an @mention inside a thread is delivered without narrowToThread being called'
  'narrowToThread has exactly one call site, inside enqueueWakeOnMessage'
  'a muted thread stays muted when the user is mentioned in it'

I confirmed the same thing from the source side earlier today on #1218: narrowToThread is called once, at agentMentionService.ts:1155, inside enqueueWakeOnMessage (1090); enqueueMentions begins at 1266 and never calls it. So "thread_root_id alone enqueues no chat.mention" is structural rather than incidental — addressing is not thread-scoped because the mention path never reaches the narrowing at all.

Nothing to change. behind = 85 against MAX_BEHIND: 40 is the only blocker, and the green Stale-base merge guard tick is expired: it fires on [opened, synchronize, reopened, edited], none of which occur when main moves underneath a PR.