refactor: clean up case routes DRY violations, remove dead export, standardize reply API

- Extract readLinkedCases() helper and resolveCasePath() to eliminate 6x duplicated
  linked-cases.json path construction and 5x duplicated file read/parse logic
- Replace O(n) .some() duplicate check with O(1) Set.has() in case listing
- Un-export isError() in types/api.ts (only used internally by getErrorMessage)
- Standardize reply.status() → reply.code() in system-routes (Fastify canonical API)
- Update CLAUDE.md: accurate frontend module listing, SSE event count (~106)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/types/api.ts

@@ -117,7 +117,7 @@ export interface CaseInfo {
  * @param value The value to check
  * @returns True if the value is an Error instance
  */
-export function isError(value: unknown): value is Error {
+function isError(value: unknown): value is Error {
   return value instanceof Error;
 }