fix(web): drop the branch action from user and steering bubbles

The branch control on user and consumed-steering bubbles could enable
only when a completed turn ended with no node after the message at all,
so readers saw a permanently unavailable control whose tooltip promised
a state it could not reach. An enabled one would mislead anyway: a fork
at a message seq cuts at the containing turn/end and takes the answer
along, the opposite of the branch-to-re-ask reading a control on one's
own bubble suggests.

MessageItem loses its fork props, PendingSteeringBubble loses the
showBranch special case, and messageBranchSeqs narrows to
assistantBranchSeqs: only a completed turn's transcript tail that is
the turn's own content-text assistant may fork. A steered turn keeps
its fork point under the settled answer, because fork is a log-prefix
cut and the steer is model-visible history the child inherits.

Web aria goldens drop the user-bubble disabled-branch row and its
hidden explanation text; the nested-subagent golden also loses the one
enabled user-tail fork handle, a loss the decision note accepts.

apps/web/tests/snapshots/markdown-inline-code-links/ui.expected.md

@@ -7,9 +7,6 @@
 - text: Show the local preview URL. {{clock}}
 - button "Copy":
   - img
-- button "Branch into a new conversation" [disabled]:
-  - img
-- text: Available only on the last message of a completed turn
 - heading "Inline code links" [level=2]
 - paragraph:
   - text: "Preview:"