wire up the adversarial integration harness

[VibhorGautam]

draft, opening early so it can grow in the open

this fills in the tests/integration stub. harness_test.go was just a doc comment and the payload file was empty, so it now builds the sidecar, starts it against a temp socket and config, and fires each payload from adversarial_payloads.json through the real process over uds, checking the decision

starting with the on_prompt hook (jailbreaks block, benign allows), green. i'll add the other three hooks and more categories from here, plus a coverage summary once there's enough to summarise

one structural call worth your eyes: tests/integration sits outside the sidecar go module, so the make integration target never actually ran. i gave it its own go.mod (path under the sidecar so it can reuse the real frame codec and signer via a replace) and pointed the makefile target at it. happy to move it inside the sidecar module instead if you'd rather that layout

otel correlation (a run emitting the same spans as prod) is a later phase and leans on #41, so i'll add it once that merges