feat: group message batching + Telegram group gating + instantGroups (#187)

* feat: add group message batching, Telegram group gating, and instantGroups

Group Message Batcher:
- New GroupBatcher buffers group chat messages and flushes on timer or @mention
- Channel-agnostic: works with any ChannelAdapter
- Configurable per-channel via groupPollIntervalMin (default: 10min, 0 = immediate)
- formatGroupBatchEnvelope formats batched messages as chat logs for the agent
- Single-message batches unwrapped to use DM-style formatMessageEnvelope

Telegram Group Gating:
- my_chat_member handler: bot leaves groups when added by unpaired users
- Groups added by paired users are auto-approved via group-store
- Group messages bypass DM pairing (middleware skips group/supergroup chats)
- Mention detection for @bot in group messages

Channel Group Support:
- All adapters: getDmPolicy() interface method
- Discord: serverId (guildId), wasMentioned, pairing bypass for guilds
- Signal: group messages bypass pairing
- Slack: wasMentioned field on messages

instantGroups Config:
- Per-channel instantGroups config to bypass batching for specific groups
- For Discord, checked against both serverId and chatId
- YAML config → env vars → parsed in main.ts → Set passed to bot

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: preserve large numeric IDs in instantGroups YAML config

Discord snowflake IDs exceed Number.MAX_SAFE_INTEGER, so YAML parses
unquoted IDs as lossy JavaScript numbers. Use the document AST to
extract the original string representation and avoid precision loss.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: Slack dmPolicy, Telegram group gating check

- Add dmPolicy to SlackConfig and wire through config/env/adapter
  (was hardcoded to 'open', now reads from config like other adapters)
- Check isGroupApproved() in Telegram middleware before processing
  group messages (approveGroup was called but never checked)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/channels/types.ts

@@ -26,6 +26,7 @@ export interface ChannelAdapter {
   // Capabilities (optional)
   supportsEditing?(): boolean;
   sendFile?(file: OutboundFile): Promise<{ messageId: string }>;
+  getDmPolicy?(): string;
   
   // Event handlers (set by bot core)
   onMessage?: (msg: InboundMessage) => Promise<void>;