The general approach without LLMs doesn't work. 50 companies have built products to do exactly what you propose here; they're called static application security testing (SAST) tools, or, colloquially, code scanners. In practice, getting every "suspicious" code pattern in a repository pointed out isn't highly valuable, because every codebase is awash in them, and few of them pan out as actual vulnerabilities (because attacker-controlled data never hits them, or because the missing security constraint is enforced somewhere else in the call chain).

Could it work with LLMs? Maybe? But there's a big open question right now about whether hyperspecific prompts make agents more effective at finding vulnerabilities (by sparing context and priming with likely problems) or less effective (by introducing path dependent attractors and also eliminating the likelihood of spotting vulnerabilities not directly in the SAST pattern book).