feat: register slash commands in Discord/Slack bots

**Epic**: #1978

Telegram registers 4 bot commands (/start, /reset, /skills, /agent). Discord and Slack have no registered commands.

**Acceptance Criteria**:
- [ ] Discord bot registers at least /start, /reset, /skills slash commands
- [ ] Slack app registers equivalent commands
- [ ] Commands appear in respective platform UI

**Complexity**: Small | **Effort**: 2h
**Ref**: CHAN-05