* 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>
When a bare Slack user ID (U-prefix) is passed as the send target
without an explicit `user:` prefix, `parseSlackTarget` classifies it as
kind="channel". `resolveChannelId` then passes it through to callers
without calling `conversations.open`.
This works for `chat.postMessage` (which tolerates user IDs), but
`files.uploadV2` delegates to `completeUploadExternal` which validates
`channel_id` against `^[CGDZ][A-Z0-9]{8,}$` — rejecting U-prefixed
IDs with `invalid_arguments`.
Fix: detect U-prefixed IDs in `resolveChannelId` regardless of the
parsed `kind`, and always resolve them via `conversations.open` to
obtain the DM channel ID (D-prefix).
Includes test coverage for bare, prefixed, and mention-style user ID
targets with file uploads, plus a channel-target negative case.
