Each top-level Slack DM now gets its own Hermes session, matching the
per-thread behavior channels already have. Previously all top-level DM
messages shared one continuous session because thread_ts was None,
causing context to accumulate across unrelated conversations.
The behavior is controlled by platforms.slack.extra.dm_top_level_threads_as_sessions
in config.yaml (default: true). Set to false to restore legacy behavior.
Based on PR #10789 by helix4u. Changes from original:
- Default flipped to true (was opt-in, now opt-out)
- Removed env var fallback (config.yaml only per project policy)
- Tests updated to cover both default and opt-out paths
bd7e272c1f