FN-7470: show Git prerequisite during onboarding

Add Git availability checks to the first-run GitHub onboarding flow so missing host prerequisites are visible before project setup.

- Probe the server-host Git CLI with a bounded core helper and expose status through the auth status API.
- Render ready/missing Git prerequisite guidance in the GitHub onboarding step with install instructions and localized strings.
- Cover the probe, auth route, and onboarding UI states with regression tests, docs, and a changeset.

Files changed:
 .changeset/fn-7470-git-onboarding.md               |  7 ++
 docs/dashboard-guide.md                            |  2 +
 docs/getting-started.md                            |  2 +-
 packages/core/src/__tests__/git-cli-status.test.ts | 83 +++++++++++++++++++++
 packages/core/src/git-cli-status.ts                | 56 ++++++++++++++
 packages/core/src/index.ts                         |  7 ++
 packages/dashboard/app/api/legacy.ts               |  8 ++
 .../app/components/ModelOnboardingModal.css        | 59 +++++++++++++++
 .../app/components/ModelOnboardingModal.tsx        | 53 ++++++++++++-
 .../__tests__/ModelOnboardingModal.test.tsx        | 87 ++++++++++++++++++++++
 .../dashboard/src/__tests__/routes-auth.test.ts    | 62 ++++++++++++++-
 .../dashboard/src/routes/register-auth-routes.ts   | 13 +++-
 packages/i18n/locales/en/app.json                  | 11 ++-
 packages/i18n/locales/es/app.json                  | 44 ++++++++++-
 packages/i18n/locales/fr/app.json                  | 44 ++++++++++-
 packages/i18n/locales/ko/app.json                  | 44 ++++++++++-
 packages/i18n/locales/zh-CN/app.json               | 44 ++++++++++-
 packages/i18n/locales/zh-TW/app.json               | 44 ++++++++++-
 packages/i18n/src/resources.d.ts                   |  9 +++
 19 files changed, 661 insertions(+), 18 deletions(-)

Fusion-Task-Id: FN-7470
Fusion-Task-Lineage: 56d6f118-0d82-4f89-bcaa-b01e72a1bf8b
Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

packages/dashboard/app/api/legacy.ts

@@ -2338,14 +2338,22 @@ export async function probeProviderModels(params: ProbeModelsParams): Promise<Pr
   });
 }
 
+export interface GitCliStatus {
+  available: boolean;
+  version?: string;
+  installUrl?: string;
+}
+
 /** Fetch authentication status for all OAuth providers */
 export function fetchAuthStatus(options?: FetchOptions): Promise<{
   providers: AuthProvider[];
   ghCli?: { available: boolean; authenticated: boolean };
+  gitCli?: GitCliStatus;
 }> {
   return dedupe("/auth/status", () => api<{
     providers: AuthProvider[];
     ghCli?: { available: boolean; authenticated: boolean };
+    gitCli?: GitCliStatus;
   }>("/auth/status"), options);
 }