feat(extensions): add sandbox extension for OS-level bash sandboxing (#673)

Uses @anthropic-ai/sandbox-runtime to enforce filesystem and network
restrictions on bash commands (sandbox-exec on macOS, bubblewrap on Linux).

Features:
- Per-project config via .pi/sandbox.json
- Global config via ~/.pi/agent/sandbox.json
- Enabled by default with sensible defaults
- --no-sandbox flag to disable
- /sandbox command to view current config

packages/coding-agent/examples/extensions/sandbox/.gitignore

@@ -0,0 +1 @@
+node_modules