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/tools/tests/scoped.spec.ts

@@ -148,6 +148,7 @@ describe('restrict()', () => {
     expect(() => ctx.tools.restrict({ deny: ['real'] })).toThrow(/requires a scoped context/)
     expect(() => scope.ctx.tools.restrict({})).toThrow(/no-op/)
     expect(() => scope.ctx.tools.restrict({ allow: ['reall'] })).toThrow(/unknown tool "reall"; known tools for this scope: real/)
+    expect(() => scope.ctx.tools.restrict({ deny: ['ghost', 'wraith'] })).toThrow(/unknown tools "ghost", "wraith"/)
   })
 })