Consolidate skills documentation into .github/skills/README.md

## Context

Per the discussion in PR #14285 ([thread](https://github.com/Azure/azure-sdk-tools/pull/14285#discussion_r2908969243)), the skills guidelines currently live in two places:

- `tools/azsdk-cli/docs/skills-guidelines.md` (legacy location, tied to the azsdk-cli)
- `.github/skills/README.md` (new location, where all shared skills now live)

## What needs to happen

1. **Merge useful content** from `tools/azsdk-cli/docs/skills-guidelines.md` into `.github/skills/README.md`
2. **Delete** the legacy `tools/azsdk-cli/docs/skills-guidelines.md` (or replace with a redirect pointer to the new location)
3. **Update any references** to the old doc path

## Why

As @benbp noted: all skills should end up in `.github/skills/` — the `azsdk-cli` location is a legacy artifact of where skills work first started. Skills should be higher-level and scenario-based, not specific/exclusive to the azsdk-cli/MCP.

## Participants

cc @samvaity @benbp @ronniegeraghty

Compare tools/azsdk-cli/docs/skills-guidelines.md with .github/skills/README.md, starting by identifying useful shared skills content to carry forward. Search the repository for references to the legacy path and update them. Done means the shared guidance is consolidated in .github/skills/README.md and the legacy document is deleted or points to the new location.

Written by the indexing model from the issue text.