Improve Components validation guidance

[javiercn]

Summary

• classify build failures before changing commands, keep validation targeted, and report untested boundaries
• require baseline-sensitive probes for transient browser behavior and document MutationObserver timing limitations

The browser guidance reflects an investigation where reading document.title from a MutationObserver callback exposed only the final state in both baseline and fix, while synchronous title-node instrumentation detected the baseline transition.

Documentation-only change; no builds or tests were run.

[Copilot]

Pull request overview

Updates developer guidance in the Components area to improve how build failures are diagnosed and how browser probes are validated when investigating transient UI behavior, emphasizing baseline verification and timing-aware instrumentation.

Changes:

• Adds guidance to classify build failures (source vs prerequisites vs infrastructure), prefer the smallest supported build, and explicitly report untested validation boundaries.
• Strengthens Playwright/browser probing guidance to require baseline-sensitive probes for transient behavior and clarifies MutationObserver timing limitations when intermediate synchronous states matter.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Components/AGENTS.md	Adds build-failure triage guidance and reinforces using the smallest build + reporting untested boundaries.
.github/skills/validate-blazor-feature/SKILL.md	Adds baseline verification and MutationObserver timing guidance for transient DOM/browser-state probes.

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