[TheGreatAxios]

Closes CL-11931

What changed

• Moved the starter mockup's <section data-screen="home"> out of workflows/designer/starter/.builder/designs/index.html into workflows/designer/starter/.builder/designs/screens/home.html, leaving designs/index.html with the shared app shell, navigation, <!-- builder:screens --> marker, and router script.
• Added spliceDesignScreens and syncDesignScreensOnDisk (apps/hub/src/builds/starter.ts, wired into apps/hub/src/builds/sidecar-tools.ts and apps/hub/src/artifacts/design-draft.ts) so every edit to .builder/designs/ re-splices designs/screens/*.html into designs/index.html on disk and in packed design bundles while omitting only the exact untouched starter home.html once another screen exists.
• Updated mockupPage (apps/web/src/pages/workspace/design-preview.tsx) and check-app (workflows/coder/check/check-app.ts) to match the hub's exact starter home.html predicate so a customized home.html that retains the starter heading and adds copy still assembles.
• Updated check_design (apps/hub/src/builds/design-check.ts and apps/hub/src/artifacts/design-tools.ts) to automatically pack and file a design_option artifact in Postgres when check-app passes, and enabled fileBeforeDone: true on the designer's compactingDirector (apps/hub/src/workflows/compaction.ts, apps/hub/src/workflows/entries/designer.ts).
• Updated workflows/designer/src/prompt.ts, workflows/designer/skills/react-ui/SKILL.md, workflows/designer/starter/README.md, and workflows/designer/README.md for per-screen files, the 3-screen cap, and automatic filing on a passing check_design.

How it was verified

• bun run format:check
• bun run lint
• bun run typecheck
• bun run test (including new tests in apps/hub/src/builds/starter.test.ts, apps/hub/src/artifacts/design-draft.test.ts, apps/hub/src/workflows/compaction.test.ts, workflows/designer/src/prompt.test.ts, and apps/web/src/pages/workspace/preview-interactions.test.tsx)
• bun run pins:verify
• bun run vendor:verify

[linear-code]

CL-11931

CL-11870

[corbits-triage]

Triage requests changes:

• Conventions: added code matches a code pattern

Nothing triage asked to change still fails, so this request no longer stands.

[TheGreatAxios]

Per-screen files assemble into the shell, and an old single-file designs/index.html bundle is left unchanged. One predicate does not match the hub.

Preview and check-app drop designs/screens/home.html when it still contains the starter heading, <h1 data-testid="home-title">Hello, world</h1>. The hub only drops that file when its bytes are exactly the untouched starter (withoutUnusedStarterHome). A real Home screen that keeps the starter heading and adds copy is omitted from <main>. check-app then says #/home has no data-screen, so check_design fails and the design cannot be filed.

An exact starter home.html should still be omitted once another screen exists. A home screen whose heading was replaced already assembles. A bundle that is only designs/index.html stays byte-identical.

[corbits-triage]

Triage requests changes:

• Conventions: added code matches a code pattern
  • apps/web/src/pages/workspace/index.tsx:1038 empty-list-fallback: Invented fallback `?? []`; handle the missing value instead.

[corbits-triage]

Triage requests changes:

• Conventions: added code matches a code pattern
  • apps/web/src/pages/workspace/index.tsx:1038 empty-list-fallback: Invented fallback `?? []`; handle the missing value instead.

Nothing triage asked to change still fails, so this request no longer stands.

[corbits-triage]

Triage requests changes:

• Merge conflicts: merge conflicts with base

[corbits-triage]

Triage: In progress

Triage is waiting for CI to finish before it decides. This comment shows the verdict once it has.