Lower follow-up composer control emphasis

[brsbl]

What was wrong

Secondary follow-up composer affordances used three different foreground tiers: the plus action inherited the primary foreground, the model caret used the muted foreground, and the collapse caret used the subtle foreground. That inconsistency gave low-priority chrome more visual weight than its function warranted and made the controls compete with the selected model and prompt content.

What changed

• Moves the composer collapse caret and plus action to the established shared subtle chrome-button treatment.
• Moves the model-picker caret to the same text-subtle-foreground/75 tier only when the picker uses its muted composer presentation; non-composer picker styling is unchanged.
• Preserves the existing hover, focus, menu, collapse, and responsive behavior.
• Adds a focused Control emphasis Ladle state without the overview story's comparison-label column.
• Adds regression assertions for all three foreground treatments. No daemon wire, SDK, API, CLI, or persisted contract changes.

Before — parent PR head f434d3f44c537a8ef943f4d4f8cc20db3ac86602

The plus action, model caret, and collapse caret render at three stronger, inconsistent foreground tiers.

After — child head 7b7f9c645bf88916298e664c444110df18cd2ff3

All three affordances use the same lower-emphasis chrome tier while model text and primary actions retain their hierarchy.

How you verified

• The new class assertions are red against the parent behavior: its computed light-theme colors are foreground for plus, muted-foreground for the model caret, and subtle-foreground for collapse. They pass at the child head with all three resolving to subtle-foreground/75.
• pnpm exec turbo run test --filter=@bb/app --force -- src/components/promptbox/PromptBoxInternal.test.tsx src/components/promptbox/PromptBoxActionsMenu.test.tsx src/components/pickers/ModelReasoningPicker.test.tsx — 144/144 tests passed.
• pnpm exec turbo run typecheck --filter=@bb/app passed on the final head.
• pnpm exec turbo run lint --filter=@bb/app completed with 0 errors; existing repository warnings remain.
• Targeted oxfmt --check and git diff --check passed.
• Chrome for Testing 152.0.7977.64 rendered the exact branch Ladle component at 390×844, 768×900, 1440×1100, and 1920×1200 with no horizontal overflow or runtime exceptions. Light and dark themes both resolved the three controls to the same subtle tier. Collapse → focus re-expansion, model-menu opening, and plus-menu opening all passed.
• Safari is not required for this non-marketing bb product UI change under the repository browser-QA policy.

BB-Thread-ID: thr_sjdd7gudiq

AGENT GENERATED