docs: clarify current comment dedup behavior

[cmondragon023]

Summary

• correct the queue job comment so it no longer claims source is persisted in ProcessedComment
• document that ProcessedComment is currently unwired and that reconciliation relies on owner-reply checks and DmLog guards
• leave runtime behavior, schema fields, and migrations unchanged

Verification

• npm run db:generate
• npx prisma validate
• npm run typecheck
• npm run lint
• npm test (184 tests)
• git diff --check upstream/main...HEAD

Closes #38

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