feat(auth): auth_service_account WASM primitive + remove TS JWT signer

- registry/components/auth_service_account: TinyGo impl for Google
  Service Account (JWT-bearer → token exchange) and base structure
  for AWS SigV4.
- .component-builds/auth_service_account: independent Worker at
  auth-service-account.arcrun.dev, extends wasi-shim with an
  http_request host function for the token exchange step.
- Delete cypher-executor/src/lib/wasm-executor.ts (legacy, replaced
  by component-loader WASM HTTP runner path).
- credential-injector.ts service_account branch now throws — all
  service_account recipes must route through auth-dispatcher.

Per .agents/specs/arcrun/credential-primitives-wasm Phase 2.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

registry/components/auth_service_account/go.mod

@@ -0,0 +1,3 @@
+module component
+
+go 1.21