Preselect the imported project, show a signed-out agent in the composer, and add shared suggestion cards

[brsbl]

Root cause

After the setup guide, the home composer drops new users into a blank state. It shows "No project" even when they just imported one. If their agent is signed out, nothing tells them until a thread fails.

Change

• Preselect the imported project. When the Projects step imports repos, the home composer selects the most recently active repo from that import.
• Show a signed-out agent inside the composer. When the selected agent on the primary machine is installed but signed out or expired, the composer's placeholder names the problem ("Codex isn't signed in", or "Your Codex session expired"). The submit button becomes Sign in, which opens the setup guide on the Agents step. There is no separate banner. The composer API for this is blockedAction on the submission config (ComposerBlockedAction, in PromptBoxInternal), so other blocked states can use it later. The guide opens through a small request atom (setup-guide-request.ts) that OnboardingGate renders as an overlay. Closing the overlay returns to the composer and does not reset onboardingCompletedAt.
• Based on Ask for notifications from a sidebar card, record setup-guide events, and expose setupComplete #5222. The shared SuggestionCard and InlineConfirmation components and their story now live in Ask for notifications from a sidebar card, record setup-guide events, and expose setupComplete #5222 with the other shared components. This PR is based on Ask for notifications from a sidebar card, record setup-guide events, and expose setupComplete #5222's branch, so its diff shows only the composer, project and setup-guide request changes.

Verification

Before: b75017c645 (#5222 head, this PR's base) · After: 183ecc0d27 (PR head) · Captured at 2×

Branch web dev app (pnpm dev) in Chrome for Testing through BB Browser Automation, same data dir for both: Codex installed with an empty CODEX_HOME (signed out), other providers disabled. For the import rows, loop-machine was deleted first, then imported alone through Settings → Show setup guide → Projects, and the home composer opened.

Screen / state	Viewport	Before	After
Home composer / Codex signed out	Desktop · 1440×900 · light
Home composer / Codex signed out	Desktop · 1440×900 · dark
Home composer / Codex signed out	Mobile · 390×844 · light
After clicking Sign in	Desktop · 1440×900 · light
Home composer / just imported loop-machine	Desktop · 1440×900 · light

The shared SuggestionCard story now lives with its component in #5222.

• Clicked Sign in in the composer: the guide opened on the Agents step. Skip setup closed it back to the composer, still in the signed-out state.
• Unit tests: mostRecentImportedProjectId, resolveComposerSignInState, and the OnboardingGate requested-step overlay. CI runs remotely.

BB-Thread: Notification card, setup-guide events, setupComplete

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