fix(slack): fetch thread context when bot is mentioned in a thread

[ygd58]

Fixes #1953

Problem

When @mentioning Hermes in a Slack thread, it had no awareness of the thread's conversation history. It could only see the current message.

Fix

When the bot is mentioned inside a thread (thread_ts != ts), fetch the last 20 messages via conversations_replies API and prepend them to the message as context:

[Thread context:]
User: test
User: @rocky can you read this thread

[New message:]

Notes

• Only fetches context when bot is mentioned in a thread (not top-level messages)
• Graceful fallback: if API call fails, proceeds without context
• Bot's own messages labeled as "Assistant", user messages labeled with their display name

[ygd58]

The test failures (ImportError: cannot import name '_rich_text_from_ansi') are pre-existing failures unrelated to this PR. The function was removed from cli.py in a previous upstream commit but the test file wasn't updated. The slack.py changes are isolated and clean.

[teknium1]

Superseded by PR #5890 which implements thread context fetching (based on the improved approach from PR #2953 by the same author). Thank you @ygd58!