fix(web): record which preset a session actually runs

The creation header names the preset a session STARTED with and is frozen,
which is correct — it is a creation fact. Switching is legal only while a
session is blank, and that looked like enough: no history exists yet.

It is not, because the switch's effect outlives the blank window. The user
switches, then sends the first message; every turn from there runs under the
new composition while the header still names the old one. The session is
then locked around a misrecorded preset, and resume reads the header to
rebuild it — composing one preset's tools over a history another produced,
which is exactly the replay the blank-only lock exists to prevent, reached
by another route. A picker showed `standard` for a session running
`core-web`.

A switch is now an `agent-preset/selected` event appended after the swap
commits, and `resolveSessionPreset()` (last selection, else the header) is
what every reconstruction reads: the summary, resume, the conflict guard,
and the fork introduced one layer down.

pnpm-lock.yaml

@@ -1113,6 +1113,9 @@ importers:
       '@deepseek-ai/dsh-client-runtime':
         specifier: workspace:^
         version: link:../../client/runtime
+      '@deepseek-ai/dsh-client-ui-agent-preset':
+        specifier: workspace:^
+        version: link:../../client/ui-agent-preset
       '@deepseek-ai/dsh-client-ui-command':
         specifier: workspace:^
         version: link:../../client/ui-command