feat: add browsable peptide directory with category filtering, REST API, and extended meta fields (v4.3.0)

Adds [peptide_directory] shortcode with card grid, category taxonomy (8 terms),
search, pagination, detail modal, and /v1/compounds REST endpoint. Includes 9
new meta fields (half-life, stability, solubility, etc.), admin data tools for
migration/re-enrichment, integration hooks for cross-plugin extensibility,
dark mode support, and full uninstall cleanup. Refactors AI generator into
PSA_OpenRouter + PSA_KB_Builder classes. Adds PHPUnit tests for PostType and
Directory classes. Fixes QA findings: dead JS selector, renderError loading
state, numeric meta validation, prepared SQL in uninstall.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -10,6 +10,11 @@ node_modules/
 .DS_Store
 Thumbs.db
 
+# Environment & credentials
+.env
+.env.local
+.env.credentials
+
 # Build artifacts
 *.zip
 *.tar.gz