[Feature] Bot Mode: allow a bot to be a member of multiple groups (each group = its own session in the bot's profile)

## Problem In Bot Mode, each bot has a single `group` field, so a bot can only be in one group chat at a time. Joining another group **moves** it out of the first (the UI is literally "Move to group…"). With several bots working a large project, all sub-project discussion is forced into one group chat. There's no way to have focused "team meetings" per sub-project — like one person attending several meetings, each focused on a specific issue. ## Request Let a bot belong to **multiple groups**. **Each group membership is its own persistent session inside the bot's profile.** The per-group `"Group: <name>"` session machinery already exists (each member bot gets its own per-group Hermes session with a separate room log and watermarks), so this is mostly a data-model change, not a new runtime concept: one bot, several rooms, separate context in each — the same way a human holds different conversations in different meetings. ## Suggested change - `meta.group` (string) → `meta.groups` (list) - Roster membership becomes: group name ∈ bot's `groups` - "Move to group…" → "Manage groups…" (add / remove) - Disband clears only that group from each member's list - Per-group sessions, watermarks, and room logs stay as-is ## Use case One org house, several sub-projects: ops / marketing / tech bots each attend the rooms relevant to them instead of one mega-chat. Keeps context per sub-project without duplicating bots or profiles. ## Note Scoped to **desktop Bot Mode** (local bots roster). Telegram multi-bot group behaviour is tracked separately (#6419, #44881).