Address PR #222 re-review: fix source-uniqueness detection + coverage/cleanups

MUST-FIX
- isSourceUniqueViolation read the wrong error field: kysely-postgres-js
  (postgres@3.4.8) puts the violated constraint on `constraint_name`, not
  node-postgres' `.constraint`, so a concurrent same-slug+language import's
  23505 was never recognized as a source-collision and surfaced a false
  "name already exists" error. Now read `constraint_name` (with `.constraint`
  as a fallback for other drivers). Fix the faked test fixture (it built the
  error with the same wrong `.constraint` field, masking the bug): it now
  uses `constraint_name`, so the test genuinely exercises the skip path and
  FAILS against the unfixed code.
- Extract the catalog modal's role-state computation into a pure
  `catalogRoleInstallState(role, workspaceRoles, language)` helper (mirrors
  role-launch.ts) and cover it with vitest: import / installed / update /
  same-slug-different-language.

SUGGESTIONS
- Restore IAiRoleUpdateFromCatalogResult as a discriminated union mirroring
  the server; narrow the consumer via `"reason" in result` (the boolean
  discriminant does not narrow under strictNullChecks:false).
- README: add a "How it's served" section documenting AI_AGENT_ROLES_CATALOG_URL
  (remote http(s) base / local path / empty => in-repo folder).
- check.mjs: drop the redundant `const key = slug` alias.
- Cover the reason->message mapping in useUpdateAiRoleFromCatalogMutation
  (4 branches) via renderHook with a mocked service.
- Cover importFromCatalog "bundle not in index" => BadGateway.
- Cover updateFromCatalog "slug in index but missing in bundle file" =>
  not-in-catalog.

ARCHITECTURE
- Extract the shared catalog read prefix: a private `loadBundleById`
  (fetchIndex -> meta -> fetchBundle -> versionMap) reused by getCatalogBundle
  and importFromCatalog, and a `catalogRoleContentFields` mapper shared by the
  import insert and update patch. The three orchestrations and their distinct
  write paths stay separate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent-roles-catalog/scripts/check.mjs

@@ -106,9 +106,8 @@ for (const bundle of bundles) {
       const where = `${bundleId}/${lang}`;
       // Only flag duplicates across DIFFERENT bundles or files; the same slug
       // is expected to appear once per language file of the same bundle.
-      const key = slug;
-      if (slugSeen.has(key)) {
-        const prev = slugSeen.get(key);
+      if (slugSeen.has(slug)) {
+        const prev = slugSeen.get(slug);
         const prevBundle = prev.split("/")[0];
         if (prevBundle !== bundleId) {
           errors.push(
@@ -116,7 +115,7 @@ for (const bundle of bundles) {
           );
         }
       } else {
-        slugSeen.set(key, where);
+        slugSeen.set(slug, where);
       }
     }
   }