Fix sym shadowing warnings + coherent softmax gating

- Rename Init() param sym -> symName in IAgent base and overrides
  (MAAgent, MomentumAgent, RegimeADX) to stop hiding the global sym.
- Softmax gating: derive temperature from dispersion of the competence
  weights ws[] (the same quantity the softmax weights), not from the
  variance of signals zs[]; skip disabled agents in the normalization.
- Use |rho| (with weightMin floor) as competence and sign-flip
  anti-correlated agents instead of zeroing them, so rho<0 information
  is used (inverted) rather than discarded.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

MQL5/Experts/MultiAgentTest/Agents/MomentumAgent.mqh

@@ -39,8 +39,8 @@ public:
          momHandle(INVALID_HANDLE), lastMomPeriod(0),
          lastZ1(0), lastZ2(0), momCorr(0.1, 5), accelCorr(0.1, 5) { signalStats.SetR(2.0); }
    
-   void Init(string sym, ENUM_TIMEFRAMES tf) override {
-      IAgent::Init(sym, tf);
+   void Init(string symName, ENUM_TIMEFRAMES tf) override {
+      IAgent::Init(symName, tf);
       momHandle = INVALID_HANDLE;
       lastMomPeriod = 0;
    }