Improve logveil chat token redaction

[rogerchappel]

What changed

• Adds a Slack-style xox* token redaction rule.
• Adds scan coverage proving the token value is removed and replaced with an indexed redaction marker.

Why it matters

Chat exports and agent logs commonly contain bot/user/app tokens. Redacting those by default makes LogVeil safer for sharing support bundles and debugging transcripts.

Verification

• npm test ✅
• npm run release:check ✅
• node /Users/roger/Developer/my-opensource/releasebox/bin/releasebox.js check . ✅

Remaining limitations

• The rule focuses on Slack-style token prefixes; other chat platforms may need additional explicit patterns later.