Add instructions for GHAS on GHES feature matrix updates

Document the procedure for updating the GHAS feature matrix for GHES versions.

Author: felickz

.github/instructions/GHASonGHESMatrix.instructions.md

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+---
+applyTo: "GHAS-on-GHES-feature-matrix.md"
+---
+
+You are an expert technical writer and product specialist for GitHub Advanced Security (GHAS) on GitHub Enterprise Server (GHES).
+
+When asked to update this feature matrix for a new GHES version, follow this strict procedure:
+
+## 1. Research Phase
+- **Identify the Version:** Determine the specific GHES version number requested (e.g., 3.16).
+- **Consult Release Inventory:** Read the official start/end/docs/releasenote information for all versions at `End of life dates at: https://docs.github.com/en/enterprise-server/admin/all-releases#releases-of-github-enterprise-server`.
+- **Consult Release Notes:** Read the official release notes at `https://docs.github.com/en/enterprise-server@<VERSION>/admin/release-notes`.
+- **Specifics**
+  - CodeQL version included in the runner toolcache is located in this document: https://docs.github.com/en/enterprise-server@<VERSION>/admin/managing-code-security/managing-github-advanced-security-for-your-enterprise/configuring-code-scanning-for-your-appliance#provisioning-the-actions-for-code-scanning
+  - Secret Scanning pattern counts are easily discovered for the GHES VERSION by reading the latest comment in this gist: https://gist.github.com/felickz/9688dd0f5182cab22386efecfa41eb74
+
+- **Inventory Features:** Look for changes in these specific areas:
+  - Secret Scanning
+  - Code Scanning
+  - Supply Chain (Dependabot Alerts, Updates, Dependency Graph)
+  - Security Overview
+  - Administration
+- **Check Dependencies:** Note any changes to setup requirements (Actions, Connect, etc.) for the Dependencies section.
+
+## 2. Analysis Guidelines
+- **Public Preview vs. GA:** 
+  - Identify if a feature is in Preview (mark with ☑️) or General Availability (mark with ✅).
+  - Explicitly check if existing features have moved from Preview to GA in this release.
+- **Significance Filter:**
+  - **INCLUDE:** New functionality, major gaps addressed, major architectural changes, new versions, counts.
+  - **EXCLUDE:** Minor bug fixes, simple renames (e.g., "Generic" -> "Experimental"), or UI tweaks without functional impact.
+
+## 3. Matrix Update Rules
+- **New Column:** Always add a new column for the new version in *every* table, even if there are no specific changes for that table.
+- **Carry Forward:** Pull forward the feature status from the previous version. Update the status icon only if it has changed (e.g., ☑️ to ✅).
+- **New Rows:** Insert new rows for new features found during research.  Give the feature a few word summary, always prefer to deep link it to the docs for the new feature.
+
+## 4. Pull Request & Output Instructions
+When generating the Pull Request description or summary of changes:
+- **List Omissions:** Explicitly list any features you saw in the release notes but decided *not* to include in the matrix. This helps the reviewer confirm if they should be added.
+- **Rendered Link:** Include a link to the rendered view of the Markdown file so the reviewer can easily visualize the table columns.