fix(coverage): exempt .d.ts files from diff coverage gate

Ambient declaration files have no runtime code so v8 coverage
never generates DA records for them. Without an allowlist entry,
coverage:diff reports no-coverage-data for every .d.ts in the
diff. Add /\.d\.ts$/ to ALLOWED_GLOBS with a companion test.

Also configure @vitest/coverage-v8 for core-shared and add
targeted vitest exclusions for infrastructure files that are not
unit-testable (DI symbols, interface files, tRPC context, Sentry
SDK init) — bringing core-shared into the L2 aggregate and making
the L1 diff gate enforce coverage on new executable code.

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

packages/core-shared/vitest.config.ts

@@ -16,8 +16,6 @@ export default mergeConfig(nodeVitestConfig, {
         "src/trpc/context.ts",
         // Sentry client init — browser/node SDK init, tested in apps
         "src/instrumentation/sentry/**",
-        // Pure type-alias file — no executable code
-        "src/payload/retention-types.ts",
       ],
     },
   },