feat(background-agent): add model-based concurrency management (#548)

* feat(config): add BackgroundTaskConfigSchema for model concurrency

🤖 GENERATED WITH ASSISTANCE OF OhMyOpenCode (https://github.com/code-yeongyu/oh-my-opencode)

* feat(background-agent): add ConcurrencyManager for model-based limits

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

* feat(background-agent): integrate ConcurrencyManager into BackgroundManager

🤖 GENERATED WITH ASSISTANCE OF OhMyOpenCode (https://github.com/code-yeongyu/oh-my-opencode)

* test(background-agent): add ConcurrencyManager tests

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

* fix(background-agent): set default concurrency to 5

🤖 Generated with [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

* feat(background-agent): support 0 as unlimited concurrency

Setting concurrency to 0 means unlimited (Infinity).
Works for defaultConcurrency, providerConcurrency, and modelConcurrency.

🤖 Generated with [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

assets/oh-my-opencode.schema.json

@@ -1658,6 +1658,35 @@
           "type": "string"
         }
       }
+    },
+    "background_task": {
+      "type": "object",
+      "properties": {
+        "defaultConcurrency": {
+          "type": "number",
+          "minimum": 1
+        },
+        "providerConcurrency": {
+          "type": "object",
+          "propertyNames": {
+            "type": "string"
+          },
+          "additionalProperties": {
+            "type": "number",
+            "minimum": 1
+          }
+        },
+        "modelConcurrency": {
+          "type": "object",
+          "propertyNames": {
+            "type": "string"
+          },
+          "additionalProperties": {
+            "type": "number",
+            "minimum": 1
+          }
+        }
+      }
     }
   }
 }