test(e2e): fix the @starter-projectss tag typo and document the tag set

Correct the single @starter-projectss -> @starter-projects typo in the extended starter-projects spec, and list the canonical tag set in tests/README so new specs reuse an existing tag instead of inventing variants.

Phase 3 (T3).

Refs LE-1665

src/frontend/tests/README.md

@@ -24,3 +24,14 @@ already covers.
 
 Prefer seeding state through `seedFlowViaApi` over rebuilding flows on the
 canvas — UI rebuilds are the suite's main CI cost.
+
+## Tags
+
+Specs are tagged (`{ tag: [...] }`) so CI can filter which suites run. Use a tag
+that already exists — don't invent variants or introduce typos (e.g.
+`@starter-projects`, never `@starter-projectss`). The tags currently in use:
+
+`@release` · `@workspace` · `@api` · `@components` · `@starter-projects` ·
+`@database` · `@deployment` · `@a11y` · `@mainpage` · `@features` · `@agents` ·
+`@folder` · `@regression` · `@notes` · `@workflow` · `@routes` · `@right-click` ·
+`@dropdown`

src/frontend/tests/extended/features/starter-projects.spec.ts

@@ -4,7 +4,7 @@ import { awaitBootstrapTest } from "../../utils/await-bootstrap-test";
 
 test(
   "user must be able to interact with starter projects",
-  { tag: ["@release", "@starter-projectss"] },
+  { tag: ["@release", "@starter-projects"] },
   async ({ page, context }) => {
     await awaitBootstrapTest(page);