Drag sidebar threads into composers to mention them

[SawyerHood]

Human comments

What was wrong

Sidebar thread drags only resolved split-pane targets, so dropping a thread onto a compose box could not insert a mention.

What changed

Register editable compose boxes as thread-mention drop targets and prioritize them over split-pane targets. Hovering shows “Mention thread”; dropping inserts the existing mention pill at the pointer position, with the normal serialized thread reference. This works in new-thread and follow-up composers while preserving split-pane drops elsewhere.

Also make the scaffold test task depend on SDK declaration generation, so packing the SDK waits for its bundled types on a clean CI checkout.

How you verified

• pnpm exec turbo run typecheck --filter=@bb/app passed.
• pnpm exec turbo run test --filter=@bb/app -- --run src/lib/split-drag/splitDragSession.test.ts src/components/promptbox/PromptBoxInternal.test.tsx passed all 149 tests, including new coverage for mention insertion, drop priority, moving away, and cancellation.
• In an isolated dev app, dragged a sidebar thread into both new-thread and follow-up composers using real browser input. Verified the highlight, correct mention token, unchanged route, and draft persistence after reload.
• pnpm exec turbo run test --filter=@bb/templates passed all 47 tests after adding the missing build dependency.
• User tested and approved the change.

AGENT GENERATED