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.

backend/explore/explore.go

@@ -9,7 +9,6 @@ import (
 	"sync"
 	"time"
 
-	"github.com/wailsapp/wails/v3/pkg/application"
 	"golang.org/x/sync/singleflight"
 
 	"yellowjacket/backend/database"
@@ -144,19 +143,9 @@ func (e *Service) CAALimiter() *RateLimiter {
 	return e.caaLimiter
 }
 
-// ServiceStartup is v3's service lifecycle hook: it runs once the
-// runtime exists, and ctx is cancelled when the app shuts down.  It
-// replaces v2's SetContext, which had to be called by hand from
-// OnStartup and was exported, so it was also bound to the frontend.
-func (e *Service) ServiceStartup(
-	ctx context.Context,
-	_ application.ServiceOptions,
-) error {
-	e.ctx = ctx
-	e.index.SetContext(ctx)
-
-	return nil
-}
+// The v3 service lifecycle hook that fills e.ctx lives in
+// servicestartup.go, behind a build tag: naming
+// application.ServiceOptions is what would drag cgo into cmd/indexbuild.
 
 // StartIndexBuild kicks off the background search index build.
 // Call this after the library scan completes so the indexer doesn't

backend/explore/servicestartup.go

@@ -0,0 +1,30 @@
+//go:build !indexbuild
+
+package explore
+
+import (
+	"context"
+
+	"github.com/wailsapp/wails/v3/pkg/application"
+)
+
+// ServiceStartup is v3's service lifecycle hook: it runs once the
+// runtime exists, and ctx is cancelled when the app shuts down.  It
+// replaces v2's SetContext, which had to be called by hand from
+// OnStartup and was exported, so it was also bound to the frontend.
+//
+// It is the one thing in this package that names the Wails application,
+// and it is behind a build tag for the reason
+// backend/events/runtime_wails.go states: cmd/indexbuild imports this
+// package and is built without cgo, GTK or WebKit.  Nothing under the
+// indexbuild tag runs a Wails app, so the hook — and the context it
+// installs — is simply absent there.
+func (e *Service) ServiceStartup(
+	ctx context.Context,
+	_ application.ServiceOptions,
+) error {
+	e.ctx = ctx
+	e.index.SetContext(ctx)
+
+	return nil
+}