docs(hubspot): add HubSpot tool README

[Nupreeth]

Description

Adds a README for the existing HubSpot integration.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

N/A

Changes Made

• Added setup instructions for HUBSPOT_ACCESS_TOKEN
• Documented the available HubSpot tools
• Added usage examples for contacts, companies, and deals

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Nupreeth]

Happy to make follow-up edits if you want this README aligned with any preferred tool-doc structure.