fix(trail): review feedback — stderr auth errors, trim type/priority, partial-update clarity
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- Route auth/not-logged-in messages to stderr (not stdout) for approve, request-changes, approvals, comment, attachment, and watchers, so '--json' output stays machine-parseable.
- Trim --type/--priority before sending on the wire (they were validated trimmed but sent raw).
- When the split metadata/body update partially applies, report that the metadata change already landed so only --body needs a retry.
- Correct the splitTrailUpdate comment to match the code's label handling.
Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com Entire-Checkpoint: 01KX6THZDP2XW3WDGFFAZ4WRXW
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