Add FeeGuard to Productivity and Workflow Tools

[decision-council]

Description

Adds FeeGuard to the "Productivity and Workflow Tools" section of README.md.

Checks Stripe Connect refunds and disputes for unreversed transfers, unadjusted application fees, and uncovered platform losses.

Disclosure: I'm affiliated with FeeGuard at Veristria.

Type of change

• Add a new resource
• Update an existing resource
• Fix formatting/typos
• Improve categorization

Checklist

• I have read contributing.md.
• My change is specifically relevant to developer essentials.
• I used the format [Name](URL) - Description. for list entries.
• I checked for duplicate entries.
• Descriptions are concise, objective, and end with a period.
• I verified links are valid and not broken.
• I used canonical HTTPS URLs without shorteners or tracking parameters.
• I disclosed any affiliation with the added/updated resource.
• This pull request contains a single logical change.

Additional context

[CLAassistant]

All committers have signed the CLA.

[decision-council]

Signed