fix(#4683): detect cross-plan threat-ID duplicates before execution (#4828)

admin_reason: missing-secondary-reviewer — self-authored overnight sweep; two orthogonal agent reviews ran (isolated adversarial REQUEST-CHANGES with all six findings dispositioned, plus a bypass/consumer-lens APPROVE) and the sha-pinned bench passed 46362/0 on the merged head.

agents/gsd-planner.md

@@ -372,6 +372,8 @@ Output: [Artifacts created]
 
 ## STRIDE Threat Register
 
+Threat IDs are unique within a phase. Before numbering, read the `<threat_model>` blocks of the phase's existing PLAN files and continue after their highest `T-{phase}-NN` — if the phase already uses `T-47-19`, new plans number from `T-47-20`, never from `T-47-01` again (#4683: a reused ID names two different threats, which makes `SECURITY.md` rows and `VALIDATION.md`'s Threat Ref column ambiguous). `T-{phase}-SC` is the exception: reserved, and every plan keeps it.
+
 | Threat ID | Category | Component | Severity | Disposition | Mitigation Plan |
 |-----------|----------|-----------|----------|-------------|-----------------|
 | T-{phase}-01 | {S/T/R/I/D/E} | {function/endpoint/file} | {critical\|high\|medium\|low} | mitigate | {specific mitigation action} |