fix(discord): resolve deferred "thinking..." indicator after slash commands

[Mibayy]

Problem

Fixes #3595.

When a slash command (e.g. /usage, /insights) was issued, the gateway called interaction.response.defer() to show the Discord "hermes is thinking..." indicator, but then dispatched the actual response through handle_message() as a regular channel message — without ever resolving the deferred interaction. This left the placeholder orphaned, so both the "thinking..." bubble and the real response appeared simultaneously.

Root Cause

_run_simple_slash defers the interaction and dispatches the command, but the previous followup.send() call only ran when followup_msg was explicitly provided. For commands that don't pass a followup_msg (e.g. /usage, /insights), nothing cleaned up the deferred placeholder at all.

Fix

After handle_message() returns, the deferred interaction is always resolved:

• With followup_msg: edit_original_response(content=followup_msg) replaces the placeholder with the confirmation text — one clean message instead of two.
• Without followup_msg: delete_original_response() removes the placeholder so the "thinking..." indicator disappears.

Both paths are wrapped in try/except so a transient Discord API error during cleanup doesn't surface to the user.

# Before
if followup_msg:
    await interaction.followup.send(followup_msg, ephemeral=True)

# After
if followup_msg:
    await interaction.edit_original_response(content=followup_msg)
else:
    await interaction.delete_original_response()

Testing

Tested by issuing /usage, /insights, /reset, and /new in a Discord server with Hermes running. The "thinking..." indicator now disappears cleanly in all cases.