readme --write cannot refresh the tagline after a description change (readme-shrink) #68

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

Part of epic: Epic: Make README generation safe for existing repositories.

Summary

After the About description changes, the default (upgrade) mode fails with readme-shrink because it treats the old generated tagline as a user line it must not drop. --mode header succeeds.

Evidence

• Reproduced on a Pick-Your-AI repository: readme --write returned fail with lost line: **Research home for…; readme --mode header --write changed exactly one line.
• The old tagline sits inside a showcase:tagline marker block.

Impact

Description edits strand stale taglines unless the operator knows the hidden header mode.

Proposed fix

Exclude content inside showcase:* marker blocks from the never-drop-a-line check.

Acceptance criteria

• Default mode refreshes the tagline; user lines are still never dropped.

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.