Add Quick Start and escalation guidance to Copilot instructions

[Copilot]

The repository had comprehensive Copilot instructions but was missing quick-reference sections recommended in GitHub's best practices guide.

Changes

• Quick Start section - Prerequisites checklist, essential validation commands (restore, build, test, lint), and critical reminders about timeouts and minimal changes
• When to Ask for Help section - Clear escalation criteria: ambiguous requirements, unresolvable build failures, security concerns, architecture decisions, and scope issues
• Best practices reference - Link to official GitHub Copilot documentation at the top

Impact

Reduces agent onboarding from scanning 400+ lines to reviewing a 30-line quick start. Provides explicit escalation paths to prevent agents from getting stuck on issues requiring human judgment.

File grew from 388 to 435 lines. All existing content preserved.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #7276

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

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