Add a New Diagnostic Code

1. Find the catalog. Grep for defineDiagnostics to locate the codes object the new entry belongs in. With several catalogs, pick by area.
2. Pick the code. PREFIX_XNNNN: PREFIX is the project name uppercased (NUXT, I18N); X is the area letter (B build, R runtime, C config, D deprecation); NNNN is the next free number. Read existing codes to choose. Never rename or reuse a published code.
3. Add the entry:

LIB_R0001: {
  why: (p: { hook: string }) => `${p.hook}() must run at the top of setup().`, // string or typed fn; becomes Error.message (required)
  fix: 'Move the call into setup() or a composable it calls.', // optional, but add whenever the fix is known
  docs: 'https://example.com/custom', // optional: overrides docsBase, or `false` to opt out
},

• why is the only required field. Params from why and fix are intersected and required at the call site.
• Runtime fields (cause, sources) are passed at the call site, never in the definition.

4. Call it: diagnostics.LIB_R0001({ hook }) to report, throw diagnostics.LIB_R0001({ hook, cause: err }) to raise. Both run the reporters.

Full API and reporter/formatter/plugin details: the nostics skill.