fix: tighten file perms and enforce Slack ACL checks (#186)

- write config and cron store with 0600 instead of 0644
- check allow list in Slack slash commands and app mentions
- pass workspace restrict flag to cron exec tool

Closes #179

pkg/channels/slack.go

@@ -296,6 +296,13 @@ func (c *SlackChannel) handleAppMention(ev *slackevents.AppMentionEvent) {
 		return
 	}
 
+	if !c.IsAllowed(ev.User) {
+		logger.DebugCF("slack", "Mention rejected by allowlist", map[string]interface{}{
+			"user_id": ev.User,
+		})
+		return
+	}
+
 	senderID := ev.User
 	channelID := ev.Channel
 	threadTS := ev.ThreadTimeStamp
@@ -345,6 +352,13 @@ func (c *SlackChannel) handleSlashCommand(event socketmode.Event) {
 		c.socketClient.Ack(*event.Request)
 	}
 
+	if !c.IsAllowed(cmd.UserID) {
+		logger.DebugCF("slack", "Slash command rejected by allowlist", map[string]interface{}{
+			"user_id": cmd.UserID,
+		})
+		return
+	}
+
 	senderID := cmd.UserID
 	channelID := cmd.ChannelID
 	chatID := channelID