Add security demo for GAIA C++ tool features

- Introduced a new interactive demo in `cpp/examples/security_demo.cpp` showcasing four modes:
  1. Tool Policies & Confirmation (ALLOW / CONFIRM / DENY)
  2. Argument Validation (validateArgs callback)
  3. Path Validation (gaia::validatePath)
  4. Shell Argument Safety (gaia::isSafeShellArg)

- Updated `docs/cpp/security.mdx` to include new sections on path validation and shell argument safety, along with examples and warnings about default policies.
- Enhanced documentation clarity on tool policies, argument validation, and security best practices.

Author: itomek

cpp/CMakeLists.txt

@@ -177,8 +177,8 @@ if(GAIA_BUILD_EXAMPLES AND WIN32)
 endif()
 
 if(GAIA_BUILD_EXAMPLES)
-    add_executable(secure_agent examples/secure_agent.cpp)
-    target_link_libraries(secure_agent PRIVATE gaia::gaia_core)
+    add_executable(security_demo examples/security_demo.cpp)
+    target_link_libraries(security_demo PRIVATE gaia::gaia_core)
 endif()
 
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