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.

packages/client/ui-agent-preset/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-agent-preset/README.md
-README.md: 322fc7c8ba6eb621f27cb09475079e3d5bccf03f
-README.zh.md: 6eece3248d7f3c3652a30579f907eaf5f888f35f
+README.md: 14921afb7b90bb0b42a8f7f83ebc78773e8419a3
+README.zh.md: 06807199e996a6ae9d8a4216b8f686b6bbc044d9

packages/client/ui-agent-preset/package.json

@@ -63,8 +63,6 @@
     "lib/index.js",
     "lib/invariant.js",
     "lib/client.js",
-    "lib/types/**/*.d.ts",
-    "lib/types/**/*.d.ts.map",
-    "src"
+    "lib/types/**/*.d.ts"
   ]
 }