feat(web): pick a session's agent preset from the composer

A seat in the composer tool row, left of the model select, showing the preset
THIS session runs — its own recorded one, not the deployment default, because a
resumed session runs what it was created with. `SessionSummary` carries
`agentPreset` for that, alongside `cwd` and `origin`.

The switch exists only while the conversation has not started. After the first
turn the seat becomes a plain label rather than a disabled menu: a greyed
control reads as "temporarily unavailable", when in fact the choice is gone for
good. The seat never asks in that state, and the host refuses independently
with `agent-preset-locked`, so a stale client cannot slip one past it.

A rejected switch restores the previous value and surfaces the host's message
rather than leaving the seat showing something the session is not running.

packages/client/ui-agent-preset/README.md

@@ -8,6 +8,12 @@ The agent-preset surface: one General-settings row choosing which [preset](../..
 
 A session's preset is fixed when the session is created — the host refuses to adopt an existing session under a different one, because that session's history was produced under the first preset's tools. So this row cannot be a live switch, and it says so: changing it applies to sessions started afterwards while running sessions keep the composition they began with.
 
+## The composer seat
+
+A second surface, in the composer tool row left of the model select: the preset THIS session runs. It shows the session's own recorded preset rather than the deployment default, because a resumed session runs what it was created with.
+
+The switch exists only while the conversation has not started. After the first turn the seat becomes a plain label — offering a disabled menu would suggest the choice is merely unavailable rather than gone. The host enforces the same rule and answers `agent-preset-locked`, so a stale client cannot slip a switch past it.
+
 ## What it reads and writes
 
 Options and the current default both come from one `agentPreset.list` call. The roster already reports which id a session with no explicit choice gets, so the row needs no settings-schema introspection; the write targets the `agent-presets` settings namespace's `default` field, which is what the host resolves at creation.
@@ -30,6 +36,5 @@ No direct invalidation. Changing the default never touches a running session's p
 
 ## Known Limitations and Deferred Work
 
-- **No per-session choice at creation** — this row sets the default only. The wire already carries `agentPreset` on `session.create`, so a session-start surface can offer the choice; that surface does not exist yet.
 - **Presets are listed by id** — a preset carries no display metadata, so the menu shows directory names.
 - **No authoring** — creating, editing, or deleting a preset is a filesystem act; this surface only chooses among what the roster supplies.