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/preset/agent-presets/src/index.ts

@@ -37,6 +37,7 @@ export {
   inactiveRows, leakedServices, livePresetMounts, mountPreset, serviceForAgent,
   unmountPresetFor, type PresetMount,
 } from './mount.ts'
+export { resolveSessionPreset, type PresetBearingSession } from './session.ts'
 export { PresetMountError, UnknownPresetError } from './types.ts'
 export type { AgentPreset, Config, PresetRoot, PresetTrust } from './types.ts'