fix: replace deprecated ephemeral flag and add error handling across modules

- Replace `ephemeral: true` with `flags: [ MessageFlags.Ephemeral ]` in all command files
- Add try/catch with logger.error to logMenteeJoin, checkAchievements, processMentorshipRole
- Extract handleIssueOpened and handlePrOpened helpers in processGitHubEvent to reduce complexity
- Add logger.error calls in all newly introduced catch blocks

src/commands/onboardMentee.ts

@@ -67,7 +67,7 @@ export const onboardMentee = async(
   const githubUsername = interaction.options.getString("github_username", true);
   const menteeUser = interaction.options.getUser("mentee", true);
 
-  await interaction.deferReply({ ephemeral: true });
+  await interaction.deferReply({ flags: [ MessageFlags.Ephemeral ] });
 
   try {
     const repoUrl = await setupMenteeRepository(amari, githubUsername);