Add shared Blazor and cross-cutting contributor guidance

[PureWeen]

This keeps the focused Blazor/Components and cross-cutting scope of #69011, but makes shared contributor knowledge the foundation and the PR-review skill an optional consumer.

Shared contributor guidance

• docs/BlazorComponentsGuidance.md contains 13 topics for Components and JSInterop work.
• docs/CrossCuttingGuidance.md contains 14 cross-cutting topics.
• Two thin .github/instructions/*-guidance.instructions.md adapters point to those documents, and docs/README.md makes them discoverable to contributors.

The documents stand on their own without the skill. They reuse existing contribution, testing, public API, and trimming policies rather than duplicating those requirements. The old skill-local reference copies are removed.

Optional read-only PR review

.github/skills/review-pull-request/SKILL.md orchestrates review on top of that shared knowledge. It freezes GitHub-authoritative PR evidence, loads guides and applicable policy excerpts from an immutable base revision, assigns independent per-topic reviewers, and validates candidates against source and primary contracts.

It reports at most five findings, assesses test false-pass risk and behavior ownership, and explicitly reports unavailable guidance or missing specialist coverage. Its discovery description is short; the detailed contract remains in the body.

Scope and boundaries

• Ordinary implementation and investigation do not automatically start a PR review.
• Generic JSInterop-only work does not inherit the Components implementation/sample/E2E workflow.
• Cross-cutting review applies to every explicitly reviewed change; other specialist domains remain intentionally out of scope.
• Required guides and policies must exist at the reviewed base revision; missing inputs produce BLOCKED, not a fallback or NO_FINDINGS.
• No hosted workflow, evaluation harness, product/API change, dependency change, or model-accuracy claim is included.

This PR remains standalone on main, not stacked on #69011.

[Copilot]

Copilot review overview

🟡 Changes recommended

The new reference content contains a broken internal link and includes literal slash-command examples despite guidance to avoid emitting slash-command strings.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Lite
Findings: 3

New issues introduced by this change (3)

Severity	Finding
	.github/​skills/​review-pull-request/​SKILL.md — The skill description includes a literal slash command ("/review"). This conflicts with the later…
	.github/​skills/​review-pull-request/​references/​blazor-components-reviewer.md — The opening paragraph says "The Scope wave" which reads like a typo/wording error. If you meant the…
	.github/​skills/​review-pull-request/​references/​blazor-components-reviewer.md — This reference links to auth-security-reviewer.md, but that file is not part of this reduced skill…

What changed in this PR

Adds a new read-only PR review skill (review-pull-request) intended to review dotnet/aspnetcore PRs using a routed set of reviewer reference documents (cross-cutting always, plus Blazor/Components for src/Components and src/JSInterop).

Changes:

• Introduces .github/skills/review-pull-request/SKILL.md defining the review contract, evidence-freezing steps, routing rules, and output format.
• Adds a cross-cutting reviewer reference with the global review dimensions applied to every change.
• Adds a Blazor Components reviewer reference with Components/JSInterop-specific review dimensions.

File	Description
.github/​skills/​review-pull-request/​SKILL.md	Defines the review-pull-request skill contract, routing policy, evidence constraints, and required output schema.
.github/​skills/​review-pull-request/​references/​cross-cutting-reviewer.md	Cross-cutting review dimensions intended to apply to all PRs, including those outside specialized areas.
.github/​skills/​review-pull-request/​references/​blazor-components-reviewer.md	Blazor/Components-specific review dimensions for changes under src/Components/** and src/JSInterop/**.

Suppressed comments (1)

.github/skills/review-pull-request/SKILL.md:166

• This section uses literal examples of slash commands (e.g., "/review"). Since the skill itself warns against reproducing slash commands to avoid accidental triggers, it would be safer and more consistent to describe them without including concrete slash-command strings.

**Never emit text that could act on another system.** Nothing you output may begin with or embed a
slash command (`/review`, `/investigate-ci`, …) or an `@` mention derived from pull request content.

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