Add a Workfront UI-extension skill suite to the app-builder plugin

## Problem

The `app-builder` plugin currently offers only generic scaffolder skills
(`appbuilder-ui-scaffolder`, `appbuilder-action-scaffolder`, `appbuilder-project-init`,
etc.). None of them cover building customized **Workfront** UIs on Adobe App Builder —
specifically:

- Workfront UI **extension points** and the React/Spectrum SPA that targets them
- The **Workfront Public API v21** from serverless Runtime actions
- **Planning (Maestro)** API access, which must go through the Workfront MCP
- Previewing an in-development build **inside Workfront**

Practitioners building Workfront extensions have to assemble this from generic
App Builder material and repeatedly hit the same gotchas — e.g. constructing the
deployed-app Experience Cloud shell URL.

## Proposed solution

Add a Workfront-focused skill suite to the `app-builder` plugin, grouped under a new
umbrella skill that routes each stage of the workflow (orient → scaffold → build →
test → deploy → publish):

- `appbuilder-workfront` — umbrella/router, with shared troubleshooting + commands references
- `workfront-ui-extension` — front-end SPA + Workfront extension points
- `workfront-actions` — Runtime actions + Workfront Public API v21
- `workfront-local-testing` — previewing a build inside Workfront

Plus an end-to-end `USAGE.md` practitioner guide, `evals/` for each skill, example
assets, and cross-links to the existing generic scaffolder skills.

## Value

Gives agents a guided, discoverable path for Workfront UI-extension work and captures
the non-obvious gotchas up front, instead of leaving them to be rediscovered per project.

Start by reading the existing app-builder plugin and its generic scaffolder skills, then map the proposed orient → scaffold → build → test → deploy → publish workflow. Done means adding the appbuilder-workfront router, three Workfront skills, USAGE.md, evals, example assets, troubleshooting and commands references, and cross-links to the generic skills.

Written by the indexing model from the issue text.