feat(ui5): Add SAP Fiori guidelines skill

[hinzzx]

What

Adds a new ui5-fiori-guidelines skill to the ui5 plugin.

It helps pick the right SAP Fiori / UI5 control for a given use case, and flags the wrong one before it ships.

Why

The most common UI5 mistake isn't syntax, it's reaching for the wrong control.

A sap.m.Bar used as an app header, a responsive table handed thousands of rows, a hand-built Dialog where a MessageBox belongs. These compile, look fine in review, then bite later as UX and performance problems.

The existing skills cover how to write UI5 well. None of them covered which control to use in the first place. This fills that gap.

What's in it

A SKILL.md router with a Wrong to Right table, a use-case to component map, and a set of short one-line rules.

For each control it gives both names, the SAPUI5 class and the ui5-* web component tag, and says so explicitly when a control only exists in one of them.

Eleven reference files under references/, read on demand, one per topic (actions, inputs, containers, navigation, lists and tables, display, messages, AI, upload, theming tokens, and a review checklist).

A REFRESH.md that documents how to re-sync the skill against SAP sources. It reports drift and proposes edits, a human reviews and applies. No unattended writes.

Notes for reviewers

The skill loads automatically through the plugin's skills/ directory scan, so no plugin.json change was needed.

Also we've reworked and restructured the below-mentioned skill a little bit, as it appears we've had similar ideas.

Credits:
https://github.com/SAP/ai-skills-library/tree/main/skills/sap-fiori-guidelines

[flovogt]

I would like to also mention "OpenUI5" in the skill. So, controls only available in SAPUI5 should be marked with "SAPUI5". Controls available in both with "OpenUI5 / SAPUI5".