* fix(slack): replace files.uploadV2 with 3-step upload flow
files.uploadV2 from @slack/web-api internally calls the deprecated
files.upload endpoint, which fails with missing_scope even when
files:write is correctly granted in the bot token scopes.
Replace with Slack's recommended 3-step upload flow:
1. files.getUploadURLExternal - get presigned URL + file_id
2. fetch(upload_url) - upload file content
3. files.completeUploadExternal - finalize & share to channel/thread
This preserves all existing behavior including thread replies via
thread_ts and caption via initial_comment.
* fix(slack): harden external upload flow and tests
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Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>
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