feat(cpt): consolidate peptide CPT with PSA; drop pr_peptide (v0.2.0) (#1)

Consolidates the `peptide` CPT under PR Core ownership, dropping the `pr_peptide` slug-collision that was 404ing all 89 peptide detail pages.

- POST_TYPE: pr_peptide -> peptide
- TAX_CATEGORY: pr_peptide_category -> peptide_category
- TAX_FAMILY + pr_peptide_family taxonomy removed
- Defensive post_type_exists/taxonomy_exists guards on both registrations
- Args harmonized as superset of PSA + prior PR Core (adds thumbnail support)
- Activation hook + init:999 version-change flush handler
- Uninstall scoped to _pr_core_authored flag (PSA-authored posts survive)
- Plugin version 0.1.1 -> 0.2.0, CHANGELOG in same commit
- ARCHITECTURE.md + CONVENTIONS.md updated with CPT ownership rule

QA gate verdict: PASS (qa-reviews/peptide-repo-core/2026-04-22-874f93b.md)
Sibling release: peptiderepo/peptide-search-ai#15 (PSA v4.5.0)

Known follow-up: manage_peptide_content capability is not granted to any role as of v0.2.0. No current UI/theme reads or writes PR Core-defined meta keys, so no regression. Follow-up thread to decide role assignment.

includes/admin/class-pr-core-admin-columns.php

@@ -2,7 +2,7 @@
 declare(strict_types=1);
 
 /**
- * Custom admin list-table columns for the pr_peptide CPT.
+ * Custom admin list-table columns for the peptide CPT.
  *
  * What: Adds evidence strength, editorial status, and dosing count columns.
  * Who calls it: PR_Core_Admin via manage_posts_columns and custom_column hooks.