- Add `model?: string` field to DiscordGuildChannelConfig and DiscordGuildEntry
- Add corresponding Zod schema validation for the new fields
- Update allow-list resolved types to include model field
- Pass channelModelOverride through the reply dispatch chain
- Apply channel model override in model selection (falls back when no session override)
- Add /model-channel command to set/clear channel model via Discord
Priority chain: channel config → guild config → agent default
Closes#3742
