feat(gateway): add native Discord thread and channel slash commands

[insecurejezza]

Summary

• add native /thread and /channel Discord slash commands
• create threads and text channels directly in the Discord adapter without agent tool use
• cover success, fallback, and failure paths with regression tests

Notes

• This replaces the tool-based approach with native Discord slash command handling.
• Thread creation first tries direct channel thread creation, then falls back to creating a starter message and opening a thread from it if Discord rejects the direct path.

Test Plan

• source /Users/jeremygavin/.hermes/hermes-agent/.venv/bin/activate
• python -m pytest -q tests/gateway/test_discord_slash_commands.py tests/gateway/test_discord_bot_filter.py tests/gateway/test_discord_free_response.py

[imadreamerboy]

duplicate #866

[teknium1]

Duplicate of #866 (identical diff). Merged via PR #1178 with modifications. Thank you @insecurejezza!