Add build infrastructure agent guidance

[PureWeen]

Add build infrastructure agent guidance

• You've read the Contributor Guide and Code of Conduct.
• You've included unit or integration tests for your change, where applicable.
• You've included inline docs for your change, where applicable.
• There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.

Ground build infrastructure changes in repository-specific instructions

Description

Adds path-scoped guidance for humans and coding agents changing ASP.NET Core's MSBuild and build infrastructure. The guidance captures concrete review lessons from #68254 without changing that PR or any build behavior:

• trace properties through wrapper scripts and bare dotnet or IDE evaluation, which can use different defaults;
• map producer to persisted/shared state to consumer and diagnostic before proposing a fix;
• locate custom-task UsingTask declarations/imports and account for registration conditions such as DesignTimeBuild;
• validate the applicable normal, direct CLI, design-time, restore/no-restore, source-build/CI, OS, architecture, and RID modes with distinct evidence scopes;
• avoid local edits under Arcade-owned eng/common and use existing build entry points with the repository SDK activated.

A source comment beside the conditional RepoTasks import records that task invocations must not run when the tasks are not registered.

No Vally eval is included. The current eng/skill-evals experiment discovers skill evals and varies environment.skills, mapping each eval to .github/skills/${eval.parent}. Attaching a path-specific .github/instructions/*.instructions.md file would require a separate instruction-overlay experiment surface, which is broader than this focused grounding change.

Validation:

• Parsed the instruction frontmatter with Ruby's YAML parser and verified the expected comma-separated applyTo patterns.
• Parsed Directory.Build.props with xmllint.
• Ran git diff --check, including an equivalent whitespace check for the instruction before it was staged.

This PR only adds guidance and a comment; it does not alter evaluated MSBuild behavior.