docs(security): replace speculative tool name with capability description

Removes `ha_manage_custom_tool` (PR #854, not yet merged, name not finalized)
and replaces it with a capability-based description to avoid naming-convention
violations on a forward reference.

Author: Patch76

SECURITY.md

@@ -15,7 +15,7 @@
 - Prompt injection paths that circumvent tool-level safeguards
   (e.g., HA entity data triggering unintended tool calls)
 - Privilege escalation within the MCP tool surface
-- Sandbox escape in `ha_manage_custom_tool` (if enabled)
+- Sandbox escape via sandboxed code execution, if that feature is enabled
 - Unintended information disclosure via API responses
 - Dependency vulnerabilities with a credible exploit path