docs: refresh setup notes and link wp-primitives v2.0.0

[aryanjasala]

Doc and setup fixes from a fresh-clone test of the skeleton.

Changes

• wp-primitives links: 40 links pointed at the v1.0.1 docs while composer.json requires ^2.0. They now point at v2.0.0; every linked file and anchor exists there. The dated validation record in docs/internal/maintenance.md keeps v1.0.1.
• Package sources: docs/getting-started.md said the packages come from GitHub. They come from Packagist and npm.
• Git hooks: npm install installs a pre-commit hook that runs lint-staged as well as the commit-message hook; both guides say so.
• Activation: wp-env activates the plugin when it starts, so the documented activate command printed "Plugin is already active". Both guides now say when the command is needed.
• Test site: docs/getting-started.md said .wp-env.json mounts the plugin in the test site too; the tests use .wp-env.tests.json.
• Jest: an initialized plugin has no Jest tests until one is added, so npm run test:js reports none; docs/local-development.md says so.
• Node: .nvmrc is 22.22.2, and engines.node is ^22.22.2 || ^24.15.0 || >=26.0.0, the range @wordpress/scripts 36 requires. npm install on Node 22.20 printed 5 EBADENGINE warnings.

Testing

• npm run lint:package-json and Prettier on package.json: pass
• Every rewritten wp-primitives link resolves at the v2.0.0 tag, anchors included