fix(gateway/slack): inject thread parent message into conversation context

[jneeee]

Summary

When Hermes receives a message inside a Slack thread, the parent (root) message that started the thread was absent from the conversation context. Only thread reply messages were visible to the agent.

Root Cause

_handle_slack_message only processes the current event payload. There was no code to fetch the thread root message via the Slack API.

Fix

Call conversations.replies with limit=1 when processing a thread reply (thread_ts present and != the message's own ts), fetch the root message, and prepend it to the agent's context as [Thread started with]: ....

Edge cases handled:

• Skip injection if the root message was posted by the bot itself (avoids recursive context loops)
• Silently ignore API errors via debug log — handler never crashes due to a missing parent
• No injection for top-level messages (ts == thread_ts) or plain DMs with no thread_ts

Changes

• gateway/platforms/slack.py: new _fetch_thread_parent() method + injection logic in _handle_slack_message
• tests/gateway/test_slack.py: new TestThreadParentInjection class with 5 tests

Tests

Test	Covers
test_parent_message_prepended_to_text	Happy path — parent injected
test_parent_fetch_failure_does_not_crash	API error → handler proceeds normally
test_bot_parent_message_not_injected	Bot-authored root message skipped
test_top_level_message_not_fetching_parent	ts == thread_ts → no API call
test_no_thread_ts_skips_fetch	Plain DM → no API call

Fixes #2950
Related to #1953

[jneeee]

Merged in #2956

Closed