chore: audit follow-ups from plan review

Four small fixes surfaced by a side-by-side plan-vs-shipped pass:

- README.md: adds the Template Catalog section the plan called out —
  links to the live site URL, the install flows (web / file / Finder),
  and templates/CONTRIBUTING.md for authors. Placed right before the
  existing Contributing section, with a catalog-specific cross-link at
  the end of that section too.
- CLAUDE.md: adds the Template Catalog section so future agent sessions
  know the regenerator pipeline exists, how it relates to release.sh +
  wiki.sh, and what the schema-sync rule is when DashboardWidget or
  ProjectTemplateManifest change.
- scarf/scarfTests/ProjectTemplateTests.swift: fixes the stale
  ProjectTemplateExampleTemplateTests docstring still referencing
  `examples/templates/` (the example moved to `templates/awizemann/`
  in 70f7cea).
- .github/workflows/validate-template-pr.yml: untangles the self-
  contradictory Python-version comment. The validator is 3.9+
  compatible; CI uses 3.11 for faster runner caching. Same stdlib
  surface, same code paths — just clearer about why.

All tests still green: 22 Swift tests in 7 suites, 16 Python tests,
catalog check passes on the site-status-checker example.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/scarfTests/ProjectTemplateTests.swift

@@ -484,9 +484,9 @@ import Foundation
     }
 }
 
-/// Validates every `.scarftemplate` shipped under `examples/templates/` in
-/// the repo. A template whose manifest, `contents` claim, or file set is
-/// out of sync will fail here — so the examples can't silently rot.
+/// Validates every `.scarftemplate` shipped under `templates/<author>/<name>/`
+/// in the repo. A template whose manifest, `contents` claim, or file set is
+/// out of sync will fail here — so shipped templates can't silently rot.
 @Suite struct ProjectTemplateExampleTemplateTests {
 
     @Test func siteStatusCheckerParsesAndPlans() throws {