.NET: docs: keep consumer skills focused on development choices - #10185

Motivation & Context

The current contributor guidance asks coding agents to update consumer skills as maintained product documentation whenever APIs, packages, samples, or integrations change. That broad trigger encourages skills to accumulate implementation details and per-change rules instead of helping developers choose an approach.

Consumer skills should guide high-level development choices with Microsoft Agent Framework. Microsoft Learn remains the documentation source for implementing those choices.

Description & Review Guide

• What are the major changes? Replace the automatic skill-update trigger with a decision-based maintenance rule. Define allowed and excluded content in the root instructions and consumer skill maintenance contract, add pointers in both language AGENTS.md files, and align both consumer SKILL.md documentation-lookup sections. Apply the same boundary to reference files, with concrete history, hosting, and authorization examples.
• What is the impact of these changes? Framework changes no longer automatically require consumer skill edits. Updates are warranted only when a recommendation becomes incorrect, a materially different choice is introduced, or a documentation link becomes invalid. Repository task skills retain their existing purpose. This PR does not rewrite existing consumer reference content or change runtime behavior.
• What do you want reviewers to focus on? Whether the scope and update criteria clearly separate development decisions from implementation documentation without restricting repository contributor guidance.

Checks: git diff --check passed; uv run --no-project python .github/tests/test_validate_agent_skill_links.py passed (16 tests); uv run --no-project python .github/scripts/validate_agent_skill_links.py skills passed. Runtime builds and unit suites were not run because this changes Markdown guidance only. Agent Skills specification validation and the GitHub publishing dry run were not run locally.

Related Issue

No linked issue. This guidance update was explicitly requested and its scope approved by Eduard van Valkenburg (@eavanvalkenburg) in the authoring conversation.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: Copilot assisted with the guidance proposal, Markdown edits, validation, and this PR description.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after a language prefix) — a workflow keeps the label and title prefix in sync automatically.