fix: add /opt/homebrew to macOS Seatbelt sandbox allowed paths (#29)

The Seatbelt sandbox profile was blocking access to Homebrew-installed
Python libraries on Apple Silicon Macs, causing dyld errors when
running AWS CLI commands via uvx.

Add /opt/homebrew to:
- SandboxConfig._default_read_paths() for Darwin
- MacOSSeatbeltBackend.PROFILE_TEMPLATE read paths

Author: alexei-led

src/aws_mcp_server/sandbox.py

@@ -161,6 +161,7 @@ def _default_read_paths() -> list[str]:
                 "/private/etc",
                 "/var/run",
                 "/dev",
+                "/opt/homebrew",
             ]
         return []
 
@@ -473,6 +474,7 @@ class MacOSSeatbeltBackend(SandboxBackend):
     (subpath "/private/var/run")
     (subpath "/var/run")
     (subpath "/dev")
+    (subpath "/opt/homebrew")
     (subpath "/Applications/Xcode.app")
     (literal "/")
     (literal "/private")