feat(web): choose the agent preset on the new-session screen

The composer seat spent nearly all its life disabled: a session's
composition is fixed once a turn has run. Move the choice to the
new-session screen beside the workspace picker, where it still works,
and let the session header report what a running session runs.

The hero pick is staged rather than applied — that screen precedes the
session it belongs to. It lands when a session becomes current and is
still blank, which covers both the session a workspace connect creates
and the blank one it reuses; riding `sessions.create` would miss the
second. It is spent on first use, matching the workspace picker.

Fix the durability the header field claimed but never had: `agentPreset`
was declared on `SessionHeader` and dropped by the JSONL header line, the
SQLite sessions row, the derived query index, and the cold list
projection, so every resumed session came back composed from nothing.

Add the web e2e lane that would have caught it — the one lane that mounts
the shipped roster, which needed `cordis:group` in the scaffold's Loader
builtins, as `mountRootInclude` already registers.

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

@@ -2,17 +2,23 @@
 
 English | [中文](README.zh.md)
 
-The agent-preset surfaces: a General-settings row choosing which [preset](../../preset/agent-presets/README.md) new sessions are composed from, a composer seat choosing this session's, and a settings section that authors the compositions themselves.
+The agent-preset surfaces: a General-settings row choosing which [preset](../../preset/agent-presets/README.md) new sessions are composed from, a chip on the new-session screen choosing the next session's, a read-only label in the session header, and a settings section that authors the compositions themselves.
 
 ## Why it is a new-session preference
 
 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
+## The new-session chip
 
-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.
+A second surface, beside the workspace picker on the new-session screen. It sits there rather than in the composer because that is where the choice is still open: a control that spends most of its life disabled belongs on the screen where it still works.
 
-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.
+The chip opens on the deployment default and its pick is *staged* — the screen precedes the session it would apply to. The stage reaches a session when one becomes current and is still blank, which covers both the session the workspace connect created and the blank one it reused; riding along on `sessions.create` would miss the second. It is spent on first use, so the next new session opens on the default again, exactly like the workspace picker beside it.
+
+A session that has started is refused rather than queued: the host answers `agent-preset-locked`, and the stage is dropped instead of waiting for a session that will never accept it.
+
+## The session-header label
+
+A third surface, beside the session title: the preset THIS session runs, as static chrome. A control there would promise a switch the host refuses outright. It reads the preset from the session's own summary — a resumed session runs what it was created with, not today's default — and resolves the display name against the same roster the General row reads.
 
 ## What it reads and writes
 
@@ -24,7 +30,7 @@ The row re-reads on `settings/changed` for its own namespace and on `connection/
 
 ## The management section
 
-A third surface, its own settings page (`settings.section` id `agent-presets`, ordered after Models — choosing a model is routine, composing an agent is the deployment-shaping act behind it): the roster as rows, and one composition open in a YAML editor at a time.
+A fourth surface, its own settings page (`settings.section` id `agent-presets`, ordered after Models — choosing a model is routine, composing an agent is the deployment-shaping act behind it): the roster as rows, and one composition open in a YAML editor at a time.
 
 A shipped preset opens read-only. It is the known-good composition a local one is written against, so reading it is the point and overwriting it is not — the deployment's copy is what a broken local preset is compared against. Authoring therefore starts by duplicating: **New preset** copies the current default, and **Duplicate** copies any row, because a copy always lands in the local root regardless of where the text came from.
 
@@ -38,7 +44,7 @@ Setting the default writes the `agent-presets` settings namespace, which the hos
 
 ## When the surfaces are absent
 
-A deployment that composes no presets answers with an empty roster, and the row, the seat, and the section all render nothing — every session then shares the host composition, and there is nothing to choose between or manage. A deployment that configures no writable root answers `authorable: false`, and the section stays a read-only browser: the rows still open, but creating is offered nowhere rather than through a button whose save always fails.
+A deployment that composes no presets answers with an empty roster, and the row, the chip, the label, and the section all render nothing — every session then shares the host composition, and there is nothing to choose between or manage. A deployment that configures no writable root answers `authorable: false`, and the section stays a read-only browser: the rows still open, but creating is offered nowhere rather than through a button whose save always fails.
 
 ## Model Experience
 
@@ -50,6 +56,6 @@ No direct invalidation. Changing the default never touches a running session's p
 
 ## Known Limitations and Deferred Work
 
-- **Presets are listed by id** — a preset carries no display metadata, so the menus and rows show directory names.
+- **A preset without metadata is listed by id** — display text is optional, and a preset that publishes none (every preset authored by duplicating another starts that way) shows its directory name.
 - **The editor is a plain textarea** — no YAML syntax highlighting, folding, or schema completion; the host's shape check on save is the only validation.
 - **A saved composition is not mounted** — a preset that parses but names a missing plugin is accepted, and fails at the next session that selects it.