feat(v0.5): export CTASK_LAUNCH_DIR into child sessions

config.EnvVars gains a 7th launchDir argument. cmd/new, cmd/resume, and
cmd/open pass ws.Meta.LaunchDir. Child sessions can read CTASK_LAUNCH_DIR
to know which subdirectory ctask launched them into.

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

cmd/new.go

@@ -119,7 +119,7 @@ func runNew(cmd *cobra.Command, args []string) error {
 		return nil
 	}
 
-	envVars := config.EnvVars(ws.Meta.Slug, ws.Meta.Mode, root, ws.Path, ws.Meta.Category, workspace.EffectiveType(ws.Meta))
+	envVars := config.EnvVars(ws.Meta.Slug, ws.Meta.Mode, root, ws.Path, ws.Meta.Category, workspace.EffectiveType(ws.Meta), ws.Meta.LaunchDir)
 
 	return session.Run(session.LaunchOpts{
 		WsDir:        ws.Path,