Remove wrong review instructinons

[JanProvaznik]

improves review agent
based on #13397

[Copilot]

Pull request overview

Updates internal contributor/reviewer guidance under .github/ by removing/streamlining some instruction sections and revising the “expert reviewer” checklist/rules.

Changes:

• Removes some markdown sections/whitespace from task/shared-code instruction docs.
• Rewrites parts of the “Backwards Compatibility” and other review-dimension guidance in expert-reviewer.md.
• Adds/adjusts guidance bullets (e.g., Microsoft.IO Redist note; updated checklist items and references).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
.github/instructions/tasks.instructions.md	Removes a bullet under “Multithreaded Task Migration” and tightens markdown layout.
.github/instructions/shared-code.instructions.md	Removes the “Related Documentation” section at the end of the shared-code instructions.
.github/agents/expert-reviewer.md	Rewords multiple review-dimension rules/checklists and adds a few new bullets/links.

Comments suppressed due to low confidence (1)

.github/instructions/shared-code.instructions.md:32

• All other instruction files under .github/instructions/ appear to include a “Related Documentation” section; removing it here makes this file inconsistent and reduces discoverability for contributors. Consider keeping the section (or adding a brief rationale if it’s intentionally omitted).

## Cross-Platform Correctness

* Handle .NET Framework vs .NET Core differences with appropriate `#if` guards.
* Environment variable access patterns differ across platforms — use the shared helpers.

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