Naming conventions: document the directory-naming policy, standardize on plugin.php entry files, fix the scaffold template

Problem

Two naming conventions have drifted, and the scaffold template contradicts one of them:

• Directory names: 27 example dirs follow <name>-<6-hex-hash>, but three don't: editor-bindings, my-first-interactive-block, post-meta-testimonial. Renaming them is not the right fix — GitHub doesn't redirect renamed tree paths, and there are live inbound links: editor-bindings from a Make Core post on Block Bindings in 6.9, and my-first-interactive-block from the Gutenberg Interactivity API quick-start guide (published to the Block Editor Handbook); its name is also the canonical create-block-interactive-template slug, which is the point of the example.
• Entry files: 27 plugins use plugin.php; 3 use a slug-named file (copyright-date-block-09aac3/copyright-date-block.php, post-meta-testimonial/post-meta-testimonial.php, server-side-render-block-d26119/server-side-render-block-d26119.php). Meanwhile the scaffold template (templates/block-examples/templates/plugin/$slug.php.mustache) generates slug-named files — so every new example is born breaking the de-facto standard.

Proposed fix

1. No directory renames. Document the policy in CONTRIBUTING.md (introduced by #149): the -<hash> suffix is legacy; new examples need a unique, descriptive slug, hash optional. The three existing non-hash dirs are grandfathered.
2. Standardize on plugin.php:
   • Update the scaffold template to generate plugin.php instead of $slug.php.
   • Rename the 3 outlier entry files to plugin.php (this changes their plugin basename, which deactivates the plugin for anyone upgrading an installed zip in place — acceptable for a teaching repo, worth a line in the release notes).
3. Record both conventions in CONTRIBUTING.md so the next example follows them without archaeology.

Out of scope

• Text domains, block namespaces, and header normalization (#150).

Read CONTRIBUTING.md and inspect templates/block-examples/templates/plugin/$slug.php.mustache first. Locate the three named slug-based entry files and update the documented directory and entry-file conventions, ensuring the scaffold generates plugin.php and all three outliers use that name; the existing directory names should remain unchanged.