Show active model in compact follow-up composer

[bb-slop-cop]

Human comments

What was wrong

The compact follow-up composer omitted its execution controls without rendering an equivalent model summary, while the container-level compact placeholder remained independently active. The verified investigation is published at https://get-bb.github.io/reports/issues/3205.html.

What changed

The prompt box now accepts a compact read-only summary, suppresses both placeholder rendering paths while that summary is visible, and retains the prompt as the editor's accessible label. The follow-up composer supplies the active model label when it is ready and empty, while drafts and blocked status text keep precedence.

How you verified

• Added a focused regression test that failed on unchanged production code because the compact editor still exposed Ask a follow-up, then passed after the fix.
• Reproduced the same failure in a second clean checkout at origin/main.
• Ran the two focused prompt-box test files: 37 passed.
• Ran the full app suite: 3,928 passed and 3 skipped across 481 files.
• Ran Turbo typecheck and lint for @bb/app; lint completed with no errors.
• Manually verified the compact Ladle story at 430 px: the model summary is visible and both visual placeholder paths are empty.
• Ran git diff --check; changed-line count is 137 text lines across 4 files (118 additions, 19 deletions).

Fixes #3205

AGENT GENERATED