fix password fields not clearing after validation error

When pasting an invalid token, the old value stayed in the input
field. Pasting a new token appended to the old one instead of
replacing it, causing repeated validation failures.

Add clearOnError: true to all 8 password prompts across setup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

setup/channels/slack.ts

@@ -154,6 +154,7 @@ async function collectBotToken(): Promise<string> {
   const answer = ensureAnswer(
     await p.password({
       message: 'Paste your Slack bot token',
+      clearOnError: true,
       validate: (v) => {
         const t = (v ?? '').trim();
         if (!t) return 'Token is required';
@@ -175,6 +176,7 @@ async function collectSigningSecret(): Promise<string> {
   const answer = ensureAnswer(
     await p.password({
       message: 'Paste your Slack signing secret',
+      clearOnError: true,
       validate: (v) => {
         const t = (v ?? '').trim();
         if (!t) return 'Signing secret is required';