feat(ui-conversation): add the New Session options slot

conversation.hero.options seats compact Session options at the right end of
the New Session row, after the workspace and agent-preset controls. It is a
session-scoped list slot rendered only while a Session exists, so plugins can
add choices such as a worktree toggle without displacing the shipped controls.

docs/subsystems/slots.md

@@ -171,7 +171,8 @@ Factory: conversation.content [ConversationContent]
 ├─ conversation.hero.brand.mark
 ├─ conversation.hero.workspace
 │  └─ conversation.hero.workspace.directoryFlow
-└─ conversation.hero.agentPreset
+├─ conversation.hero.agentPreset
+└─ conversation.hero.options
 
 Factory: conversation.chat.user.content [UserContent]
 ├─ conversation.chat.user.images [MessageImages]

docs/subsystems/slots.zh.md

@@ -173,7 +173,8 @@ Factory: conversation.content [ConversationContent]
 ├─ conversation.hero.brand.mark
 ├─ conversation.hero.workspace
 │  └─ conversation.hero.workspace.directoryFlow
-└─ conversation.hero.agentPreset
+├─ conversation.hero.agentPreset
+└─ conversation.hero.options
 
 Factory: conversation.chat.user.content [UserContent]
 ├─ conversation.chat.user.images [MessageImages]