feat(onboarding): fresh installs start with detected providers + welcome card

[robinebers]

Approved issue

Part of #626 (first-launch onboarding) — delivers the minimal first-launch experience; contextual tips remain open.

TL;DR

Fresh installs no longer light up all nine providers. First launch detects which AI tools actually have credentials on the Mac and enables only those (falling back to Claude, Codex, Cursor), with a one-time dismissible welcome card pointing at Customize. Existing installs are completely untouched.

What was happening

• Every provider was enabled by default, so new users got a wall of menu-bar icons for tools they don't use — a recurring complaint.
• Enablement was stored as a disabled list, so every future provider we add would also pop up enabled for everyone.
• There was no onboarding at all: nothing explained where Customize lives or that providers can be hidden.

What this changes

• First-run seeding (FirstRunSeeder): on a genuinely fresh install (empty defaults domain, checked before migrations run), Claude/Codex/Cursor are enabled immediately, then each provider's local credentials are probed asynchronously and the enabled set switches to exactly the detected providers. No detections → the starter trio stays. A toggle the user flips during the probe wins over detection.
• Local credential detection: ProviderRuntime gains hasLocalCredentials(); every provider implements it by reusing the same auth-store loading (config files, Keychain, SQLite, env keys) as its refresh() — local checks only, no network calls.
• Enabled-list storage: fresh installs persist openusage.enabledProviders.v1 (providers not in the list are off), so future providers default to off for them. Installs with existing settings keep the legacy disabled-list semantics — their setup and upgrade behavior don't change.
• Welcome card: a one-time dismissible card at the top of the dashboard ("Welcome to OpenUsage") with an Open Customize button. It only disappears via its ✕ button and never returns after dismissal.
• Docs updated (dashboard.md, menu-bar.md, settings.md, adding-a-provider.md, AGENTS.md) — including the new provider checklist item for hasLocalCredentials().

Heads-up

• The detection probe is best-effort: a provider whose credentials appear after first launch isn't auto-enabled later; the user enables it in Customize (the welcome card points there).
• New providers added later default to off for fresh-install users but remain on for legacy-mode users, matching each mode's semantics.

Tests

• New FirstRunSeederTests (sync fallback, async detection, empty detection, upgrade no-op, idempotency, user-toggle precedence, hint persistence) and expanded ProviderEnablementStoreTests for the enabled-list mode.
• swift test passes locally; manual verification with a wiped defaults domain (fresh install seeds + shows card) and with existing defaults (nothing changes, no card).

Screenshots

First launch on a Mac where only Claude was detected:

Made with Cursor

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