[Bug]: Slack test message fans out to every bound conversation instead of a selected test channel

[openworkaibot]

Title: [Bug]: Slack test message fans out to every bound conversation instead of a selected test channel

Summary: *
In Messaging > Advanced > Send test message, leaving Peer ID empty for Slack sends the test message to every Slack conversation/thread bound to the workspace directory. This creates spam across old Slack threads; Slack test sends should target a single configurable test channel instead.

Steps to reproduce: *

1. Configure a Slack identity in OpenWork and interact with the bot from multiple Slack channels or threads so they are bound to the same workspace directory.
2. Open Messaging > Advanced > Send test message.
3. Select Slack, leave Peer ID empty, keep the default directory, enter Test, and click Send test message.
4. Check the Slack conversations where the bot has previously interacted.

Expected behavior: *
Slack test sends should target one explicit destination, not fan out to every bound conversation. The UI should let the user choose a channel, default to #internal-bot-testing (C0AP6KJ0NTS), allow that default to be changed, and explain how to copy a channel ID from the Slack web app URL.

Actual behavior: *
OpenWork dispatches the test message to all Slack bindings for the selected directory, so every active or historical bot conversation/thread in that workspace can receive Test.

** Likely affected areas:**
packages/app/src/app/pages/identities.tsx:1319
packages/server/src/server.ts:2055
packages/opencode-router/src/bridge.ts:966.

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Label: bug