fix(api): prefer workspace Slack links in prompts

[decofe]

Summary

• update Slack formatting instructions to prefer workspace-hosted Slack archive URLs when present in the thread
• keep slack.com/archives as a fallback when no workspace host is available

Prompted by: @gakonst

Test

• uv run --project services/api pytest services/api/tests/test_integration.py::TestBuildSessionContext

[Zygimantass]

We've merged the Rust rewrite in #344, meaning the old API and Slackbot services are deprecated. As such we're closing this PR - if you think this is a mistake, please reopen the PR and ping me.