add empty-array IDOR FP and OAST source-IP SSRF FP signals

Two single-bullet additions to the False Positives sections of two
existing skills:

- idor.md: empty array / null returned for another user's resource
  is silent enforcement (not exposure) — compare against the owner's
  view to confirm
- ssrf.md: OAST callback whose source IP matches the tester's machine
  was made client-side (browser / client fetch), not by the server

Both signals surfaced from the false-positive-indicators content
originally proposed in this PR; rest of the PR's scope (test scaffolding,
confidence module, system-prompt protocol, deprecated .jinja prompt
format) was dropped because the .jinja vulnerability prompt format has
been replaced by the .md skills and the other parts didn't have
production consumers.

strix/skills/vulnerabilities/idor.md

@@ -187,6 +187,7 @@ query IDOR {
 - Soft-privatized data where content is already public
 - Idempotent metadata lookups that do not reveal sensitive content
 - Correct row-level checks enforced across all channels
+- Empty array / null returned for another user's resource — silent enforcement, not exposure; compare against the owner's view to confirm the data is actually missing rather than just hidden from the response shape
 
 ## Impact
 

strix/skills/vulnerabilities/ssrf.md

@@ -157,6 +157,7 @@ Server-Side Request Forgery enables the server to reach networks and services th
 - Strict allowlists with DNS pinning and no redirect following
 - SSRF simulators/mocks returning canned responses without real egress
 - Blocked egress confirmed by uniform errors across all targets and protocols
+- OAST callbacks where the source IP matches the tester's machine, not the server — the browser or a client-side fetch made the request, not the backend
 
 ## Impact