Continue shared CFG library migration: fix prologue/epilogue, implicit fields, cleanup

Agent-Logs-Url: https://github.com/github/codeql/sessions/011f8465-5ad3-44e2-9d84-f5314ea1afb2

Co-authored-by: owen-mc <62447351+owen-mc@users.noreply.github.com>

Author: Copilot

go/ql/lib/change-notes/2026-03-30-shared-cfg-library.md

@@ -0,0 +1,4 @@
+---
+category: fix
+---
+* The Go control flow graph implementation has been migrated to use the shared CFG library. This is an internal change with no user-visible API changes.