Add WordPress ecosystem note to distinctiveness principle

[glacierphonk]

Summary

• Adds a WordPress-specific note after the existing Wordfence exception in Section 7 ("Distinctiveness Over Descriptiveness") of principles.md
• The wp.org plugin directory is search-driven, creating pressure toward descriptive plugin names (Advanced Custom Fields, Contact Form 7)
• But the most memorable WordPress plugins are distinctive (Yoast, Elementor, Jetpack, Akismet) — their discoverability comes from tagline/description fields, not keyword-stuffing the name
• One paragraph, inserted between the existing exception and the "AI prefix trap" subsection

Test plan

• markdownlint-cli2 '**/*.md' passes (0 errors)

Closes #166