Add update-release-graph skill

[richlander]

Adds a new Copilot skill for updating the HAL+JSON release information graph.

What this does

Documents the workflow for regenerating the release graph using the dotnet-release tool's new graph generation commands:

• dotnet-release generate indexes — runs all generators in one shot
• dotnet-release generate version-index — root, major, patch, manifest, SDK, downloads indexes
• dotnet-release generate timeline-index — timeline root, year, month indexes
• dotnet-release generate llms-index — AI-optimized entry point

Skill covers

• Source data conventions — releases.json, release.json, _manifest.json, and how they feed the generators
• Graph topology — version hierarchy, timeline hierarchy, shortcut links, mutability rules
• Workflows — new patch release, new major version, EOL updates
• Validation — dotnet-release verify releases, markdownlint, spot-check scripts
• Common mistakes — hand-editing generated files, wrong generator order, missing source data

Relationship to other skills

Skill	Scope
update-release-graph (this PR)	HAL+JSON graph files (index.json, manifest.json, llms.json, downloads, timeline)
update-supported-os	supported-os.json/md
update-os-packages	os-packages.json/md
update-distro-packages	distros/ directory
cve-enricher	timeline CVE data (cve.json)