fix(slack): slash commands broken in DM threads due to session key mismatch

[drafish]

Problem

Slash commands (/usage, /stop, !cmd etc.) are broken inside Slack DM threads — they get treated as regular text messages and forwarded to the agent instead of being intercepted as commands.

Root Cause

_has_active_session_for_thread() (line 2869 in gateway/platforms/slack.py) hardcodes chat_type="group" when building the session key for lookup:

source = SessionSource(
    platform=Platform.SLACK,
    chat_id=channel_id,
    chat_type="group",  # ← always "group", even for DMs
    user_id=user_id,
    thread_id=thread_ts,
)

This generates a key like agent:main:slack:group:D0ASJ8Q7TDG:{user_id}:{thread_ts}, but the actual DM session key is agent:main:slack:dm:D0ASJ8Q7TDG:{thread_ts}. The keys never match, so the function always returns False for DM threads.

Impact chain:

1. _has_active_session_for_thread() returns False → thread context is prepended to every message
2. text becomes [Thread context — ...]\n/usage instead of /usage
3. get_command() checks text.startswith("/") → False → returns None
4. Gateway treats the message as regular user text → forwards to the agent

This affects all slash commands in DM threads, not just the ! prefix workaround.

Fix

Detect DM vs group from the channel ID (Slack DM channel IDs start with "D"):

chat_type="dm" if str(channel_id).startswith("D") else "group",

Testing

Verified locally: after the fix, !usage in a DM thread is correctly intercepted as a command and returns the usage report instead of being forwarded to the agent.

[teknium1]

Thanks for isolating a real current-main session-key mismatch: _has_active_session_for_thread() still builds a group source at plugins/platforms/slack/adapter.py:4015-4021, while inbound IM/MPIM messages create dm sources at plugins/platforms/slack/adapter.py:2763,3163-3170.

Problems

• The proposed D-prefix rule is incomplete. Main treats both im and mpim as DM-style sessions (plugins/platforms/slack/adapter.py:2763), while the test suite uses G_MPIM for an MPIM (tests/gateway/test_slack_mention.py:369-374). A non-D MPIM would still be looked up as group.
• The PR adds no regression test. Existing lookup coverage is group-only (tests/gateway/test_slack_approval_buttons.py:618-641).

Suggested changes

• During salvage, port this to plugins/platforms/slack/adapter.py and pass the event-derived is_dm/chat type into _has_active_session_for_thread() at both call sites (:2858, :2888) instead of re-inferring it from the ID.
• Add an existing-DM-thread test with fetched context and a command, plus an MPIM case.

Automated hermes-sweeper review.

[teknium1]

Merged via #69479 — your commit was cherry-picked/reapplied onto current main with your authorship preserved in git history: your DM-thread session-key fix was cherry-picked directly.

Thanks for the contribution!