fix(dsh-plugin-browserskill): register observation routes once webServer appears

ctx.get('webServer') at apply time raced the web composition's service
registration (the fallback SPA then answered /bsk-observation/* with
index.html). Ride ctx.inject(['webServer'], …) instead: the callback
runs when the service is provided and never runs in headless
compositions, keeping route registration timing-safe and optional.

packages/dsh-plugin-browserskill/tests/dispose.test.ts

@@ -65,6 +65,8 @@ describe("dispose cleanup ownership", () => {
     const ctx = {
       tools: { register: (def: ToolDefinition) => tools.set(def.name, def) },
       get: () => undefined,
+      // No webServer in this harness: the inject callback never runs.
+      inject: () => {},
       effect: (fn: () => () => Promise<void>) => {
         disposers.push(fn());
       },