refactor: delete dead Go code (R1)

- internal/httpapi/stubs.go: delete — 5-line comment-only orphan file with
  no declarations; its own comment said the stubs live in phase3.go.
- internal/notifier/notifier.go: delete VerifyApprovalToken — zero call
  sites; phase3.go:DecideApproval reimplements the check inline (noted as
  dead in docs/mbse). hashToken stays (used by generateApprovalToken).
- internal/checkdefaults/defaults.go: unexport ResolveHost, ForEntityType,
  ShortSlug, DefaultInterval — only called within the package. Ensure stays
  exported (called by internal/db/seed.go).

go vet, go build, and affected tests pass.

internal/httpapi/stubs.go

@@ -1,6 +0,0 @@
-package httpapi
-
-// Remaining stubs for endpoints that depend on tables not yet created
-// (knowledge_entities, agent_activity). These are kept here because the
-// phase3.go file already defines them; this file is deliberately empty.
-// The stubs live in phase3.go as simple errNotImplemented returns.