Use “violation” instead of “diagnostic” for convention failures

Convention failures are called both “diagnostics” and “violations.” Standardize on “violation” for failed convention predicates while retaining warning and error as violation severity values; ordinary operational errors should continue to be called errors and remain conceptually separate from error-severity violations.

Relevant files:

• packages/konsistent/src/core/diagnostics.ts
• packages/konsistent/src/core/reporter.ts
• packages/konsistent/src/commands/check.ts
• packages/konsistent/README.md
• docs/reference/cli.md

Rename diagnostic-oriented types, functions, fields, flags, output terminology, tests, and documentation to violation-oriented equivalents. Make operational error paths explicit enough that “error” cannot be mistaken for a convention violation without its severity context.

Backward compatibility: internal names, CLI terminology, and documentation can be renamed or removed without compatibility shims. No konsistent.json schema change is expected; if one becomes necessary, retain the old name and behavior as deprecated and emit a non-failing CLI warning when it is used.