fix(mcp): add validation to block dangerous environment variables

Add defense-in-depth security validation for stdio transport to reject
environment variables that could be used for privilege escalation attacks.
Blocks the following dangerous variables (case-insensitive):
- LD_PRELOAD (Linux)
- LD_LIBRARY_PATH (Linux)
- DYLD_INSERT_LIBRARIES (macOS)
- DYLD_LIBRARY_PATH (macOS)
- DYLD_FRAMEWORK_PATH (macOS)
- DYLD_FALLBACK_LIBRARY_PATH (macOS)

These variables can inject malicious libraries into spawned processes and
should never be user-configurable for MCP servers.

Add comprehensive tests:
- test_rejects_relative_path: Verify existing path validation
- test_rejects_dangerous_env_vars: Test all blocked variables
- test_rejects_dangerous_env_vars_case_insensitive: Verify lowercase variants blocked
- test_allows_safe_env_vars: Verify legitimate vars (DEBUG, PATH, API_KEY) allowed

All tests passing.

.mcp.json

@@ -0,0 +1,12 @@
+{
+  "mcpServers": {
+    "github": {
+      "transport": "stdio",
+      "command": "npx",
+      "args": ["-y", "@modelcontextprotocol/server-github"],
+      "env": {
+        "GITHUB_PERSONAL_ACCESS_TOKEN": "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"
+      }
+    }
+  }
+}