docs: create bot platform feature matrix documentation

## Summary

The runtime includes bridges for 9 IM platforms (Telegram, Slack, Discord, DingTalk, WeChat, WeCom, Feishu, QQ) but there is no documentation listing supported platforms, their feature parity, or setup instructions.

## Current State

- **packages/runtime/src/bots/**: 9 platform bridges exist
- **packages/runtime/src/bots/types.ts:106-117**: sendTypingIndicator and startReplyStream are optional methods
- **packages/runtime/src/bots/bot-registry.ts:106-114**: Graceful fallback for unimplemented methods

## Missing Documentation

1. Which platforms are supported and their maturity level
2. Feature matrix: streaming replies, typing indicators, ephemeral messages, file attachments
3. Setup instructions per platform (bot token creation, webhook configuration)
4. Known limitations per platform
5. Security considerations (webhook verification, token storage)

## Proposed Solution

Create docs/bot-platforms.md with:
- Platform support matrix table
- Setup guide per platform
- Feature parity documentation
- Known limitations and workarounds

## Acceptance Criteria

- [ ] Documentation covers all 9 supported platforms
- [ ] Feature matrix is accurate and up-to-date
- [ ] Setup instructions are tested and working

## Labels

enhancement, documentation