fix(build): keep the index tools free of the Wails application

The v3 migration put application.Get() in backend/events and a
ServiceStartup hook in backend/explore, both of which cmd/indexbuild
reaches. v3's application package is GTK/WebKit bindings on Linux, so
the index-artifact job — a plain golang container with CGO_ENABLED=0,
on the stated grounds that neither command imports the app — stopped
compiling with "undefined: pointer". That job owns the ~205 GB dump
checkpoint, so it is the worst place to learn this.

Both are behind the indexbuild tag now: the one app.Event.Emit lives in
runtime_wails.go, runtime_indexbuild.go answers ErrNoRuntime (what the
app itself returns before Run, so Deliver's callers need no second
path), and explore's ServiceStartup moves to its own tagged file.

TestIndexToolsDoNotImportWails walks `go list -deps -tags indexbuild`
so the claim the workflow makes is checked rather than assumed.

.gitea/workflows/index-artifact.yml

@@ -42,7 +42,10 @@ jobs:
     runs-on: ubuntu-latest
     container:
       # CGO is not needed: the project uses the pure-Go modernc sqlite
-      # driver, and neither command imports the Wails app.
+      # driver, and neither command imports the Wails app — which is a
+      # claim with a test behind it now (cmd/indexbuild/deps_test.go),
+      # because the v3 migration quietly broke it and this job was where
+      # that surfaced.
       image: golang:1.25
       # This host path must exist on the runner and be listed verbatim in
       # act_runner's container.valid_volumes. It holds explore-staging/