fix(slack): split reply threading defaults by surface

[jackberger03]

Summary

• split Slack reply threading defaults by surface: DMs default to direct replies, channels keep threaded replies
• preserve legacy reply_in_thread as an override for both surfaces
• include Slack chat type/message id in thread metadata so synthetic top-level threads are not mistaken for real threads by progress/final sends

Linear

• OTO-497

Tests

• python -m pytest tests/gateway/test_slack.py::TestProgressMessageThread::test_resolve_thread_ts_splits_dm_and_channel_defaults tests/gateway/test_slack.py::TestProgressMessageThread::test_resolve_thread_ts_honors_existing_threads tests/gateway/test_slack.py::TestProgressMessageThread::test_resolve_thread_ts_legacy_reply_in_thread_overrides_both_surfaces tests/gateway/test_slack_mention.py::test_config_bridges_slack_reply_in_thread_surface_knobs -q -o 'addopts='
• python -m pytest tests/gateway/test_slack.py tests/gateway/test_slack_mention.py tests/gateway/test_config.py tests/gateway/test_config_env_bridge_authority.py -q -o 'addopts='

Note: this is a salvage of the closed PR #30088 approach; commit authorship is credited to the original author.

[teknium1]

Thanks for preserving the original Slack threading work. The underlying behavior is still present on current main: top-level DMs receive a synthetic thread_ts at plugins/platforms/slack/adapter.py:2781-2784, and _resolve_thread_ts returns the metadata thread id by default at plugins/platforms/slack/adapter.py:1661-1666.

Problems

• The diff targets gateway/platforms/slack.py, but Slack was migrated to plugins/platforms/slack/adapter.py in 560010547. The old adapter path is no longer the runtime implementation.
• The proposed Slack-specific additions in gateway/config.py and gateway/run.py conflict with the current plugin ownership: Slack YAML translation now lives in plugins/platforms/slack/adapter.py:_apply_yaml_config (4485-4519), and thread metadata is generic in gateway/run.py:14357-14401.

Suggested changes

• Port the resolver and its send call sites to plugins/platforms/slack/adapter.py.
• Make the two new settings plugin-owned extras while retaining legacy reply_in_thread precedence.
• Update coverage for direct DM final/progress sends through the current plugin path.

Automated hermes-sweeper review.

[teknium1]

Current main migrated this adapter to plugins/platforms/slack/adapter.py in 560010547; this path is no longer the runtime Slack implementation. Please port this resolver change and its send call sites to the bundled Slack plugin.

[teknium1]

Merged via #70196 — your commit was cherry-picked/reapplied onto current main with your authorship preserved in git history: your surface-split reply-threading defaults were cherry-picked.

Thanks for the contribution!