This commit adds more granular control over group activation with new modes:
- 'replies': Bot only responds when someone replies to one of its messages
- 'mention+replies': Bot responds to @mentions OR replies (most flexible)
- 'never': Bot never responds except for control commands from owners
The existing 'mention' (default) and 'always' modes remain unchanged.
**Implementation details:**
- Added new GroupActivationMode types: 'replies', 'mention+replies', 'never'
- Enhanced group-gating logic to detect replies to bot messages
- Updated all command help messages and documentation
- Reply detection uses both JID and E.164 matching for reliability
**Use cases:**
- 'replies': Reduces noise in busy groups, bot only joins when explicitly engaged
- 'mention+replies': Allows natural conversation flow after initial @mention
- 'never': Temporarily disable bot in a group without removing it
Closes: N/A (feature request)
