feat: load from plugins (#12864)

Support loading plugins.

Plugins can now be enabled via [plugins.<name>] in config.toml. They are
loaded as first-class entities through PluginsManager, and their default
skills/ and .mcp.json contributions are integrated into the existing
skills and MCP flows.

codex-rs/core/config.schema.json

@@ -370,6 +370,9 @@
             "personality": {
               "type": "boolean"
             },
+            "plugins": {
+              "type": "boolean"
+            },
             "powershell_utf8": {
               "type": "boolean"
             },
@@ -1085,6 +1088,22 @@
       ],
       "type": "string"
     },
+    "PluginConfig": {
+      "additionalProperties": false,
+      "properties": {
+        "enabled": {
+          "default": true,
+          "type": "boolean"
+        },
+        "path": {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        }
+      },
+      "required": [
+        "path"
+      ],
+      "type": "object"
+    },
     "ProjectConfig": {
       "additionalProperties": false,
       "properties": {
@@ -1718,6 +1737,9 @@
         "personality": {
           "type": "boolean"
         },
+        "plugins": {
+          "type": "boolean"
+        },
         "powershell_utf8": {
           "type": "boolean"
         },
@@ -2015,6 +2037,14 @@
     "plan_mode_reasoning_effort": {
       "$ref": "#/definitions/ReasoningEffort"
     },
+    "plugins": {
+      "additionalProperties": {
+        "$ref": "#/definitions/PluginConfig"
+      },
+      "default": {},
+      "description": "User-level plugin config entries keyed by plugin name.",
+      "type": "object"
+    },
     "profile": {
       "description": "Profile to use from the `profiles` map.",
       "type": "string"