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/package.json

@@ -0,0 +1,19 @@
+{
+	"name": "pi-extension-sandbox",
+	"private": true,
+	"version": "1.0.0",
+	"type": "module",
+	"scripts": {
+		"clean": "echo 'nothing to clean'",
+		"build": "echo 'nothing to build'",
+		"check": "echo 'nothing to check'"
+	},
+	"pi": {
+		"extensions": [
+			"./index.ts"
+		]
+	},
+	"dependencies": {
+		"@anthropic-ai/sandbox-runtime": "^0.0.26"
+	}
+}