[codex] Record external agent import results (#28396)

## Summary
- restore `externalAgentConfig/import/progress` notifications while
keeping `externalAgentConfig/import/completed` as the must-deliver event
- persist completed external-agent config imports in state DB by
`importId`, including concrete success/failure details for config,
AGENTS.md, skills, plugins, MCP servers, subagents, hooks, commands, and
sessions
- add `externalAgentConfig/import/readHistories` so clients can recover
persisted import results after missing the live completion notification
- include `errorType` on import failures in protocol
responses/notifications and persisted DB JSON so future code can
classify failures without another wire/storage shape change

## Validation
- `git diff --check`
- `just test -p codex-state external_agent_config_imports`
- `just test -p codex-app-server-protocol`
- `CODEX_SQLITE_HOME=/private/tmp/codex-app-server-sqlite-read-details
just test -p codex-app-server
external_agent_config_import_sends_completion_notification_for_sync_only_import`

Also ran earlier broader checks before publishing:
- `just test -p codex-state`
-
`CODEX_SQLITE_HOME=/private/tmp/codex-app-server-external-agent-test-sqlite
just test -p codex-app-server external_agent_config`
- `just test -p codex-external-agent-migration`

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

@@ -0,0 +1,127 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "ExternalAgentConfigImportItemTypeFailure": {
+      "properties": {
+        "cwd": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "errorType": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "failureStage": {
+          "type": "string"
+        },
+        "itemType": {
+          "$ref": "#/definitions/ExternalAgentConfigMigrationItemType"
+        },
+        "message": {
+          "type": "string"
+        },
+        "source": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "failureStage",
+        "itemType",
+        "message"
+      ],
+      "type": "object"
+    },
+    "ExternalAgentConfigImportItemTypeSuccess": {
+      "properties": {
+        "cwd": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "itemType": {
+          "$ref": "#/definitions/ExternalAgentConfigMigrationItemType"
+        },
+        "source": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "target": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "itemType"
+      ],
+      "type": "object"
+    },
+    "ExternalAgentConfigImportTypeResult": {
+      "properties": {
+        "failures": {
+          "items": {
+            "$ref": "#/definitions/ExternalAgentConfigImportItemTypeFailure"
+          },
+          "type": "array"
+        },
+        "itemType": {
+          "$ref": "#/definitions/ExternalAgentConfigMigrationItemType"
+        },
+        "successes": {
+          "items": {
+            "$ref": "#/definitions/ExternalAgentConfigImportItemTypeSuccess"
+          },
+          "type": "array"
+        }
+      },
+      "required": [
+        "failures",
+        "itemType",
+        "successes"
+      ],
+      "type": "object"
+    },
+    "ExternalAgentConfigMigrationItemType": {
+      "enum": [
+        "AGENTS_MD",
+        "CONFIG",
+        "SKILLS",
+        "PLUGINS",
+        "MCP_SERVER_CONFIG",
+        "SUBAGENTS",
+        "HOOKS",
+        "COMMANDS",
+        "SESSIONS"
+      ],
+      "type": "string"
+    }
+  },
+  "properties": {
+    "importId": {
+      "type": "string"
+    },
+    "itemTypeResults": {
+      "items": {
+        "$ref": "#/definitions/ExternalAgentConfigImportTypeResult"
+      },
+      "type": "array"
+    }
+  },
+  "required": [
+    "importId",
+    "itemTypeResults"
+  ],
+  "title": "ExternalAgentConfigImportProgressNotification",
+  "type": "object"
+}