fix(slack): honor disable_dms setting

[vexclawx31]

Summary

• ignore incoming Slack DM message events when slack.disable_dms or SLACK_DISABLE_DMS is enabled
• ignore Slack slash commands invoked from DM channels under the same setting
• keep channel/group mention routing unchanged

Why

Slack channel allowlists intentionally do not filter DMs, but operators may configure external/project-scoped agents to avoid DMs. The existing disable_dms setting could be present in config/env without being enforced by the Slack adapter.

Test plan

• /Users/vex/.hermes/hermes-agent/venv/bin/python -m pytest tests/gateway/test_slack.py -k 'disable_dms or dm_slash_command_keeps_dm_session_semantics or plain_slash_still_works' -q -o 'addopts='
• python3 -m py_compile plugins/platforms/slack/adapter.py tests/gateway/test_slack.py
• git diff --check

[teknium1]

Thanks for tracing the missing configuration bridge; the underlying 1:1-DM gap is present on current main.

Problems

• plugins/platforms/slack/adapter.py:2527 gates on is_dm, which includes both im and mpim. That suppresses MPIM group-DM messages before normal routing. Current documentation defines MPIMs as shared surfaces that retain channel-style mention and allowlist controls (website/docs/user-guide/messaging/slack.md:419), and current main distinguishes one-to-one DMs with is_one_to_one_dm (plugins/platforms/slack/adapter.py:2835).
• The new slash-command check only recognizes D-prefixed channels, so MPIM message events and slash commands would be handled inconsistently.

Suggested changes

• Limit this setting to channel_type == "im" / is_one_to_one_dm, then add an MPIM regression test proving group routing is unchanged.
• Document slack.disable_dms in the Slack messaging guide.

Automated hermes-sweeper review.

[teknium1]

is_dm includes mpim, so this also drops group-DM messages before their established shared-surface mention/allowlist routing. Please gate only one-to-one im DMs (or consistently extend the policy and document/test MPIM behavior); the slash-command path currently recognizes only D IDs.

[teknium1]

Merged via #70196 — your commit was cherry-picked/reapplied onto current main with your authorship preserved in git history: your disable_dms enforcement was cherry-picked.

Thanks for the contribution!