Fix Slack and Discord thinking message replacement

[sadfun]

Summary

• show one thinking block while Slack and Discord turns are in progress
• replace that same message with the complete answer when the turn finishes
• reuse the original message for the first chunk and post only unavoidable overflow
• fall back to posting the complete answer when the final edit fails
• add shared draft-stream regression coverage

Verification

• bun test
• bun run check
• bun run build

Greploop

• Greptile confidence: 5/5
• unresolved Greptile comments: 0

[greptile-apps]

Greptile Summary

This PR changes Slack and Discord draft streaming so the thinking message is replaced by the completed answer rather than showing a partial streamed preview.

• Keeps one progress placeholder until completion.
• Replaces that placeholder with the first completed-answer chunk and posts overflow chunks separately.
• Falls back to posting the complete answer when replacement fails.
• Adds shared regression coverage for replacement, overflow, failure, and empty-answer behavior.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/channels/draft-stream.ts	Reworks the draft lifecycle to retain a progress placeholder until completion and then replace it with final answer text, resolving the previously reported duplicate streamed-answer failure.
test/draft-stream.test.ts	Adds focused coverage for successful replacement, suppressed partial previews, overflow chunking, replacement failure, and completion without an answer.

_{Reviews (2): Last reviewed commit: "address greptile review feedback (greplo..." | Re-trigger Greptile}

[sadfun]

@greptile review