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/message-actions/ui.expected.md

@@ -8,9 +8,6 @@
 - button "Copy":
   - img
 - tooltip "Copy"
-- button "Branch into a new conversation" [disabled]:
-  - img
-- text: Available only on the last message of a completed turn
 - button "Think The user wants me to read a.txt and b.txt, then reply with \"DONE\". Let me do both reads in parallel.":
   - img
   - img
@@ -38,9 +35,6 @@
 - text: Stopped Now give the final answer. 7/25 {{clock}}
 - button "Copy":
   - img
-- button "Branch into a new conversation" [disabled]:
-  - img
-- text: Available only on the last message of a completed turn
 - paragraph: DONE
 - button "Copy":
   - img