test: close the per-file coverage gaps for the scoping surface

Every subject-extractor row of the invariants carrier table is exercised
with a matching and a foreign-keyed carrier; the HMR re-apply seed path
(sessions of agents that predate the plugin are marked started) is pinned;
the scoped tool-provider disposal, plural restrict() validation, singular
scopeHost absentee, tool-subagent passthrough, stale-stage drop, and
disposing-parent spawn (INACTIVE_EFFECT, no orphan) each gain their test.
Two genuinely defensive branches carry justified v8-ignore markers.

packages/core/scope/tests/scope.spec.ts

@@ -227,4 +227,9 @@ describe('scopeHost', () => {
     await expect(scopeHost(ctx, ['tools', 'systemPrompt']))
       .rejects.toThrow('scopeHost: services "tools", "systemPrompt" not available')
   })
+
+  it('names a single absent service in the singular', async () => {
+    const ctx = new Context()
+    await expect(scopeHost(ctx, ['tools'])).rejects.toThrow('scopeHost: service "tools" not available')
+  })
 })