gusto: AI-optimize MCP action set

Gusto has no actions built in Pipedream today — there's nothing an agent can do with this integration yet. We'd like to build an initial action set so an agent can manage payroll, employee records, and time-off requests, and so each tool is easy for an agent to configure.

A few things an agent can't do today, for example:
- Look up an employee's profile or employment details
- Check or update someone's time-off balance or request
- Run or check the status of a payroll run
- List a company's departments or employees
- Look up compensation or benefits details for an employee

**API reference:** https://docs.gusto.com/app-integrations/docs/introduction

Note: Gusto requires a review before granting API access — see the linked sub-issue for details.

**Add marker:**
When a component is AI-optimized, add this as the first line of the file:
```
// x-pd-ai: optimized
```
Add it to each action file and to the app's `.mjs` file.

Start with the Gusto API reference and the linked sub-issue about API access review. Define the initial payroll, employee, department, time-off, compensation, and benefits actions, then add them as action files alongside the app's .mjs file. Done means the requested actions exist, are agent-configurable, and each action file plus the app file begins with the specified AI-optimization marker.

Written by the indexing model from the issue text.