feat(web): draft a custom preset in a creator-mode session

The settings section gains the conversational authoring entry beside
copying: a dashed add-card (the Models page's affordance) that stages the
self-referential cordis preset and starts a new session on it, closing the
settings panel through the shell's new section owner-prop `close` — the
one shell affordance a section receives, for flows that leave settings.

The seat learns the difference between picking and staging: `select()`'s
immediate apply meets the still-current running session and drops the
stage as unservable, so the entry uses `stage()` and leaves the apply to
the list-change applier that fires when the started session becomes
current. `load()` stops regressing the display when its reply lands after
an applied stage was consumed — staged pick first, then the composition
the current session already carries, then the deployment default.

The authoring lane connects a workspace and drives the gesture to a
composed host session; the section goldens gain the entry.

apps/web/tests/snapshots/agent-preset-authoring/created.expected.md

@@ -76,3 +76,6 @@
       - paragraph:
         - text: 预设文件：
         - code: {{presetRoot}}/my-agent
+  - button "用「创造模式」创作自定义预设":
+    - img
+    - text: 用「创造模式」创作自定义预设

apps/web/tests/snapshots/agent-preset-authoring/section.expected.md

@@ -58,3 +58,6 @@
       - 'button "复制: 创造模式"':
         - img
         - text: 复制
+  - button "用「创造模式」创作自定义预设":
+    - img
+    - text: 用「创造模式」创作自定义预设