LOW: Extract shared splitMessage and formatItemsForPrompt utilities

[Brad-Edwards]

Severity: Low (Quality L1, L3)

Duplicated code:

• `splitMessage` — nearly identical in `bot/bot.ts` and `delivery/telegram.ts` with slightly different heuristics
• `formatItemsForPrompt` — nearly identical in `briefing.ts` and `research.ts`

Recommendation: Extract shared utilities and use in both places.