feat(discord): add /thread command, auto_thread config, and media metadata fix (#1178)

- Add /thread slash command that creates a Discord thread and starts a
  new Hermes session in it. The starter message (if provided) becomes
  the first user input in the new session.
- Add discord.auto_thread config option (DISCORD_AUTO_THREAD env var):
  when enabled, every message in a text channel automatically creates
  a thread, allowing parallel isolated sessions.
- Fix Discord media method signatures to accept metadata kwarg
  (send_voice, send_image_file, send_image) — prevents TypeError
  when the base adapter passes platform metadata.
- Fix test mock isolation: add app_commands and ForumChannel to
  discord mocks so tests pass in full-suite runs.

Based on PRs #866 and #1109 by insecurejezza, modified per review:
removed /channel command (unsafe), added auto_thread feature,
made /thread dispatch new sessions.

Co-authored-by: insecurejezza <insecurejezza@users.noreply.github.com>

gateway/config.py

@@ -304,6 +304,8 @@ def load_gateway_config() -> GatewayConfig:
                     if isinstance(frc, list):
                         frc = ",".join(str(v) for v in frc)
                     os.environ["DISCORD_FREE_RESPONSE_CHANNELS"] = str(frc)
+                if "auto_thread" in discord_cfg and not os.getenv("DISCORD_AUTO_THREAD"):
+                    os.environ["DISCORD_AUTO_THREAD"] = str(discord_cfg["auto_thread"]).lower()
     except Exception:
         pass