[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/ServerNotification.json

@@ -1167,6 +1167,12 @@
             "null"
           ]
         },
+        "errorType": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "failureStage": {
           "type": "string"
         },
@@ -1219,6 +1225,24 @@
       ],
       "type": "object"
     },
+    "ExternalAgentConfigImportProgressNotification": {
+      "properties": {
+        "importId": {
+          "type": "string"
+        },
+        "itemTypeResults": {
+          "items": {
+            "$ref": "#/definitions/ExternalAgentConfigImportTypeResult"
+          },
+          "type": "array"
+        }
+      },
+      "required": [
+        "importId",
+        "itemTypeResults"
+      ],
+      "type": "object"
+    },
     "ExternalAgentConfigImportTypeResult": {
       "properties": {
         "failures": {
@@ -6333,6 +6357,26 @@
       "title": "RemoteControl/status/changedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "externalAgentConfig/import/progress"
+          ],
+          "title": "ExternalAgentConfig/import/progressNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ExternalAgentConfigImportProgressNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "ExternalAgentConfig/import/progressNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {