fix(slack): recover context for orphan task updates

[Choquer-agency]

Summary

• Detect short top-level Slack updates that likely refer to a task/thread, like "she paid".
• Fetch recent channel context and nearby thread context before passing the message to the agent.
• Instruct the agent to ask a grounded confirmation instead of updating tickets from ambiguous pronouns.

Test Plan

• ./venv/bin/python -m pytest tests/gateway/test_slack.py::TestSlackOrphanContextUpdates tests/gateway/test_slack_mention.py::test_config_bridges_slack_reply_in_thread -q
• ./venv/bin/python -m py_compile gateway/platforms/slack.py tests/gateway/test_slack.py

[teknium1]

Thanks for identifying the lost-context case. The premise still holds on current main: top-level DMs are keyed to their own ts at plugins/platforms/slack/adapter.py:2781-2784, while context recovery currently runs only for actual thread replies at plugins/platforms/slack/adapter.py:2886-2900.

Problems

• The PR edits gateway/platforms/slack.py, which was moved to plugins/platforms/slack/adapter.py by 5600105478ff; this requires a port, not a clean cherry-pick.
• The new renderer would inject recent historical messages without the current [unverified] sender tagging and guidance used by _fetch_thread_context at plugins/platforms/slack/adapter.py:3803-3839.

Suggested changes

• Port the feature into the bundled Slack plugin and populate MessageEvent.channel_context at plugins/platforms/slack/adapter.py:3214-3226; gateway/run.py:10391-10395 already consumes that field in the intended order.
• Reuse the current authorization/tagging behavior for every recovered historical message and test an unauthorized sender.

Automated hermes-sweeper review.