shell: add background flag to tool schema

data/tools/shell.meta

@@ -4,7 +4,9 @@
   "prompt": "Execute a single bash command in a sandboxed environment.\n\nUsage: {\"cmd\": \"your command here\"}\n\nSandbox prevents dangerous operations. Use for computation, builds, scripting.\ntimeout applies to each call (default: 30s). A non-zero exit is an error.",
   "args": {
     "type": "object",
-    "required": ["cmd"],
+    "required": [
+      "cmd"
+    ],
     "properties": {
       "cmd": {
         "type": "string",
@@ -21,9 +23,13 @@
       "bypass": {
         "type": "boolean",
         "description": "Run outside the sandbox via bypass broker."
+      },
+      "background": {
+        "type": "boolean",
+        "description": "Run in background. Returns PID immediately; output is injected automatically."
       }
     }
   },
   "tier": "hot",
   "scope": "global"
-}
+}