docs(getting-started): add first-run troubleshooting for credentials …

[Jahnavi-Krishna]

Description

This PR adds a small First-Run Troubleshooting subsection to the README.md Quick Start flow.

The goal is to reduce first-run evaluation friction by surfacing a few setup blockers that are easy to hit before users start debugging agent behavior, including:

• provider credits / quota exhaustion
• missing required tools or integrations
• credential-dependent sample workflows
• account-specific Google OAuth/admin restrictions
• provider or credential issues that can look like product failures on first run

Type of Change

• Documentation update

Related Issues

None linked. This PR is based on first-run onboarding friction observed during live testing.

Changes Made

• Added a new ### First-Run Troubleshooting subsection under ## Quick Start in README.md
• Documented common first-run blockers related to provider credits/quota, missing tools, and credential-dependent workflows
• Added guidance that some Google account setups may hit OAuth/admin restrictions and that provider/credential issues can appear like product failures during initial evaluation

Checklist

• I have performed a self-review of my changes
• I have made corresponding changes to the documentation
• My changes generate no new warnings