Add al-appsource-review leaf (wire into al-code-review)

[JesperSchulz]

Goal

Add the al-appsource-review leaf so al-code-review reviews the appsource domain, matching every other domain that already has a review leaf (11 today: breaking-changes, error-handling, events, interfaces, performance, privacy, security, style, ui, upgrade, web-services).

Build

• New action skill microsoft/skills/review/al-appsource-review.md, modelled on the existing al-*-review.md leaves and the DO composition contract (skills/do.md).
• Frontmatter: inputs: [pr-diff], outputs: [findings-report], no sub-skills (leaf).
• Source the microsoft/knowledge/appsource/ folder; emit findings with references[] back to those articles (e.g. affix/AS0011, ApplicationArea/AS0062).
• Register it in al-code-review's sub-skills list — the structural R26 check enforces set-equality between the review folder's leaves and the super-skill registry.

Acceptance

• Leaf authored + registered in al-code-review sub-skills.
• python .github/scripts/validate_frontmatter.py --root . → 0/0 (incl. R26 set-equality).
• pwsh .github/scripts/Test-KnowledgeIndex.ps1 -Root . → PASSED.
• Findings cite appsource knowledge articles.

Refs

Part of review-leaf coverage (tracker + Epic). The appsource knowledge landed in #65 (currently open) — this should land after it. Template: any existing al-*-review.md.