docs(scf): replace the stale legacy plan reference (#246)

Author: s6pa1rta3n-lab

docs/SCF_TRANCHE_PLAN.md

@@ -153,7 +153,7 @@ The plan fits naturally into the SCF three-tranche structure with:
    on hardening, audit, and governance readiness as required by SCF.
 
 5. **No duplication**: This document adds new tranche structure without
-duplicating large sections from existing docs like `docs/SCF_PLAN.md`; link
+duplicating large sections from existing docs like the [platform plan](./PLATFORM_PLAN.md); link
 out for detailed background.
 
 ## Version Control