fix: hold the session read-only while it moves into a fork

An edit in the session's preview during a move saves a draft in the
workspace being left, after the move listed what it brings, at any of
its awaits. The session now reads as read-only for the whole move, so
the preview shows its items without an editor until the move ends.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/copilot/chat/AIChatManager.svelte.ts

@@ -785,6 +785,9 @@ export class AIChatManager implements ChatViewHost {
 	/** Use, Plan or Build. A session keeps its own: the runtime seeds it from the session record
 	 * and stores each change through `onWorkModeChanged`. */
 	workMode = $state<AIWorkMode>(AIWorkMode.BUILD)
+	/** The session is moving into a fork. Its editors stay closed meanwhile: an edit would save a
+	 * draft in the workspace being left, after the move listed what it brings along. */
+	moving = $state(false)
 	onWorkModeChanged: ((mode: AIWorkMode) => void) | undefined = undefined
 	workModesAvailable = $derived(this.isSessionChat && supportsPlanMode(this.mode))
 	/** Use is the only mode: the workspace is protected, or the user's role builds nothing in it. */

frontend/src/lib/components/sessions/SessionWrapper.svelte

@@ -541,6 +541,7 @@
 			return
 		}
 		moving = true
+		if (runtime) runtime.manager.moving = true
 		const releaseAutoResume = runtime?.manager.holdAutoResume()
 		// Bracketed like a turn, so another tab on this session cannot send while its drafts
 		// move, then re-reads the session (its new workspace, the model note) once it ends.
@@ -614,6 +615,7 @@
 			sendUserToast(`Session moved to ${req.targetName}`)
 		} finally {
 			moving = false
+			if (runtime) runtime.manager.moving = false
 			releaseAutoResume?.()
 			if (chatId)
 				localRunEnded(session.id, runtime?.manager.historyManager.getCurrentChatId() ?? chatId)

frontend/src/lib/components/sessions/sessionPermissions.svelte.ts

@@ -42,7 +42,7 @@ export function useSessionReadOnly(workspace: () => string | undefined) {
 }
 
 /** The session chat whose work mode the session UI follows. */
-type WorkModeHost = { readonly workMode: AIWorkMode }
+type WorkModeHost = { readonly workMode: AIWorkMode; readonly moving?: boolean }
 
 /**
  * What the session UI may offer in `workspace`. Its editors save drafts, so it reads
@@ -51,7 +51,7 @@ type WorkModeHost = { readonly workMode: AIWorkMode }
  * resolves: hiding every editor from a developer while `whoami` is in flight is the worse
  * mistake, and the server refuses an operator anyway. A session held in Use opens no editor: one
  * in a protected workspace, one whose role builds nothing there, or one whose chat is in Use
- * mode — the chat `chat` returns, else the `aiChatManager` context's.
+ * mode — the chat `chat` returns, else the `aiChatManager` context's — and one moving into a fork.
  * Reads context and registers an effect: call during component initialisation.
  */
 export function useSessionPermissions(
@@ -62,10 +62,12 @@ export function useSessionPermissions(
 	const rights = useEditRights(workspace)
 	const user = useActingUser(workspace)
 	const protectedWorkspace = useSessionReadOnly(workspace)
+	const host = $derived(chat?.() ?? contextChat)
 	const readOnly = $derived(
 		protectedWorkspace.current ||
 			!buildsAnything(rights.capabilities) ||
-			(chat?.() ?? contextChat)?.workMode === AIWorkMode.USE
+			host?.workMode === AIWorkMode.USE ||
+			!!host?.moving
 	)
 	const workspaces = fromStore(userWorkspaces)
 	const entry = $derived(workspaces.current.find((w) => w.id === workspace()))