content icon prompt UX: invalid size_class is not explained, and the prompt asks for things generators cannot do #61

Status: Verified during the Pick Your AI rollout (reproduced, or read directly in the code).

Part of epic: Epic: Make the repo-icon pipeline reliable and honest.

Summary

An invalid size_class such as medium returns only missing: ["SIZE_CLASS"]; the valid values (WIDE, TALL, SQUARE, DIAGONAL) are only in source. The prompt also asks for "four candidates" and "seed 1025 when supported", which Codex image generation does not support.

Evidence

• Reproduced: content icon prompt --option A --glyph with size_class: medium prints only the missing slot name.
• The prompt text contains "Produce four candidates. Use seed 1025 when supported."; Codex returned one image per call.

Impact

Trial and error to supply the glyph file; provenance claims a seed that was never applied.

Proposed fix

List valid values in the error; word the prompt for single-image generators and record what was actually requested.

Acceptance criteria

• The error names the allowed values.
• The prompt does not promise candidates or seeds that the generator ignores.

Tasks

Each task below is a sub-issue.

Prior work

Prior work: open PR #20 (opt-in --allow-t3 and the Pick Your AI family), open #7 and PR #13 (variant J badges), merged PRs #14 to #19 (icon art-direction path, UTF-8 output, registry-free org health and label planning, Windows test portability, footer URL repair) and closed issues #8 to #12. Nothing here duplicates those; where an item is already handled it is linked, not refiled.

Fresh VM verification: not run. Findings were verified on a Windows host; hosted CI is separate evidence.