Support external agent config detect and import (#12660)

Migration Behavior

* Config
  *  Migrates settings.json into config.toml
* Only adds fields when config.toml is missing, or when those fields are
missing from the existing file
  *  Supported mappings:
    env -> shell_environment_policy
     sandbox.enabled = true -> sandbox_mode = "workspace-write"

* Skills
  *  Copies home and repo .claude/skills into .agents/skills
  *  Existing skill directories are not overwritten
  *  SKILL.md content is rewritten from Claude-related terms to Codex

* AgentsMd
  *  Repo only
  *  Migrates CLAUDE.md into AGENTS.md
* Detect/import only proceed when AGENTS.md is missing or present but
empty
  *  Content is rewritten from Claude-related terms to Codex

codex-rs/app-server-protocol/schema/json/v2/ExternalAgentConfigDetectResponse.json

@@ -0,0 +1,49 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "ExternalAgentConfigMigrationItem": {
+      "properties": {
+        "cwd": {
+          "description": "Null or empty means home-scoped migration; non-empty means repo-scoped migration.",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "description": {
+          "type": "string"
+        },
+        "itemType": {
+          "$ref": "#/definitions/ExternalAgentConfigMigrationItemType"
+        }
+      },
+      "required": [
+        "description",
+        "itemType"
+      ],
+      "type": "object"
+    },
+    "ExternalAgentConfigMigrationItemType": {
+      "enum": [
+        "AGENTS_MD",
+        "CONFIG",
+        "SKILLS",
+        "MCP_SERVER_CONFIG"
+      ],
+      "type": "string"
+    }
+  },
+  "properties": {
+    "items": {
+      "items": {
+        "$ref": "#/definitions/ExternalAgentConfigMigrationItem"
+      },
+      "type": "array"
+    }
+  },
+  "required": [
+    "items"
+  ],
+  "title": "ExternalAgentConfigDetectResponse",
+  "type": "object"
+}