Simplify workflow review instructions

[mfoltz]

Motivation

• Make the workflow-review guidance more concise and durable so reviewers focus on trigger correctness, brittle logic, and diagnosability rather than one-off remediation details.
• Remove PyYAML-specific remediation notes and replace them with a short, general rule about YAML validation to reduce maintenance and false negatives in reviews.

Description

• Update .github/instructions/workflows.instructions.md to keep core reviewer priorities (trigger correctness, path filters, checkout depth/history assumptions, merge-base logic, matrix behavior, and diagnosability) and the rule to avoid style-only comments.
• Reword the bootstrap guidance to advise running bash .codex/install.sh before local workflow or tooling checks.
• Remove the PyYAML example and replace it with a concise rule to Use an appropriate YAML-aware validator for workflow files and to treat missing local validator dependencies as an environment/tooling limitation rather than a workflow syntax failure.

Testing

• No automated tests were run because this is a documentation-only change.

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Codex Task

[Copilot]

Pull request overview

This PR streamlines the repository’s workflow-review guidance to focus reviewers on durable correctness and diagnosability checks, while removing PyYAML-specific local validation instructions.

Changes:

• Rewords local setup guidance to recommend running bash .codex/install.sh before performing local workflow/tooling checks.
• Replaces PyYAML parsing examples with a general rule to use an appropriate YAML-aware validator for workflow files.
• Clarifies that missing local YAML validation dependencies should be treated as an environment/tooling limitation, not a workflow syntax failure.