Remind agents to use Simplified Technical English in PR and issue templates #1986

AI agents and contributors write most PR descriptions and issue reports. The templates should remind the author to use Simplified Technical English (STE): short sentences, plain words, active voice, one idea per sentence.

Scope:

• Add an STE reminder to the pull request template (header note and a pre-submission checklist item).
• Add an STE reminder to every issue template (Markdown templates and YAML issue forms).
• No other template content changes.