Standardize convention reference terminology on vendor names

The <vendor>/<name> reference syntax calls its first component a vendor in validation messages but a source prefix or prefix in implementation and documentation. This should have one name so references and conventionSources are explained consistently.

Relevant files:

• packages/konsistent/src/config/schema.ts
• packages/konsistent/src/config/source-resolver.ts
• packages/konsistent/src/config/reference-expander.ts
• docs/reference/reusable-conventions.md
• docs/reference/configuration.md

Use “vendor” or “vendor name” throughout identifiers, types, validation messages, and documentation. Describe conventionSources as mapping vendor names to convention sources and preserve <vendor>/<convention-name> as the canonical reference form.

Backward compatibility: internal names and documentation can be renamed or removed without compatibility shims. No konsistent.json key or behavior needs to change; if a schema-facing rename is introduced, retain the old form as deprecated and emit a non-failing CLI warning when it is used.