fix: post Slack sends to other channels instead of capturing into active thread

[mslipper]

The live-delivery capture in _capture_live_slack_send was swallowing slack.send_message calls that targeted a different channel and folding them into the active thread reply. The guard for "is this bound for the active thread?" only bailed when the channel was a Slack ID that differed, so a different channel referenced by name (or with thread_ts omitted) was wrongly captured — the agent would report it had posted nothing to the requested channel.

This inverts the logic to opt-in: capture only when the request is unambiguously bound for the active thread (empty/matching channel and empty/matching thread). Since the active channel from the thread_key is always a Slack ID, a channel name now correctly fails the match and posts normally. Adds TestCaptureLiveSlackSend covering the active-thread, different-ID, different-name, and different-thread cases.

[Zygimantass]

We've merged the Rust rewrite in #344, meaning the old API and Slackbot services are deprecated. As such we're closing this PR - if you think this is a mistake, please reopen the PR and ping me.