fix: remove provisional workspace when launch is canceled with no changes

When `ctask new` launched the agent or shell and the child exited without
touching any files (e.g. the user canceled at Claude Code's trust prompt),
the workspace was left behind as empty clutter. Now, if the invocation
just created the workspace and the manifest diff is empty (zero added,
modified, deleted), the workspace directory is removed and finalize is
skipped. resume/open/last are unaffected (NewlyCreated defaults to false),
and --no-launch is unaffected (session.Run is never called).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/new.go

@@ -122,11 +122,12 @@ func runNew(cmd *cobra.Command, args []string) error {
 	envVars := config.EnvVars(ws.Meta.Slug, ws.Meta.Mode, root, ws.Path, ws.Meta.Category, workspace.EffectiveType(ws.Meta))
 
 	return session.Run(session.LaunchOpts{
-		WsDir:   ws.Path,
-		EnvVars: envVars,
-		Agent:   agent,
-		Mode:    ws.Meta.Mode,
-		Slug:    ws.Meta.Slug,
-		Shell:   newShell,
+		WsDir:        ws.Path,
+		EnvVars:      envVars,
+		Agent:        agent,
+		Mode:         ws.Meta.Mode,
+		Slug:         ws.Meta.Slug,
+		Shell:        newShell,
+		NewlyCreated: true,
 	})
 }