Don't re-trigger on Slack message edits

[sentry-junior]

Junior re-triggers and executes a new response when a Slack message it already responded to is edited/updated.

• Current behavior: editing a message that previously invoked Junior causes Junior to process it again as a new command
• Expected behavior: Junior should not respond to message_changed events for messages it has already handled
• Impact: unintended duplicate actions or responses when users correct typos or append context to prior messages

Fix likely involves tracking which message timestamps have already been handled and ignoring subsequent update events for those.

Action taken on behalf of Michi Hoffmann.